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  • American Institute of Physics (AIP)  (154,286)
  • 101
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    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 116 (2002), S. 3800-3804 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The adsorption of 1,4-cyclohexadiene on the Si(001) surface is studied by first-principles density-functional calculations within the generalized gradient approximation. The "pedestal" structure where the two C(Double Bond)C double bonds react with different Si dimers is found to be more stable than the "upright" structure where only one of the two C(Double Bond)C bonds reacts with a Si dimer. However, the [2+2] cycloaddition reaction can easily form the upright structure but not the pedestal one. The latter structure can be obtained from the former through a high energy barrier of ∼0.95 eV, indicating a small reaction rate at room temperature. Our results provide the theoretical basis for the interpretation of recent low-energy electron diffraction and photoelectron spectroscopy data in which the upright structure was seen. © 2002 American Institute of Physics.
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  • 102
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    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 116 (2002), S. 3795-3799 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We report results from a high-resolution, incoherent inelastic neutron scattering (IINS) study of xenon hydrate. This study extends previous work in which the existence of a strong coupling between localized guest vibrations and the lattice modes was shown1 for the first time [Tse et al., Europhys. Lett., 54, 354 (2001)]. This guest–host coupling might be responsible for the glass-like temperature dependence of the thermal conductivity of the crystalline gas hydrates. Our experiment focused on the low-frequency phonon density of states of the ice-like water lattice of xenon hydrate. We found two broad maxima in the density of states (DOS) at energy transfers of 7.3 and 10.3 meV. The first peak is assigned to the transverse acoustic (TA) phonons near the zone boundary and the second to the fold-back of the TA modes towards the zone center. The guest–host coupling could be confirmed by finding three distinct low energy peaks in the DOS at energy transfers of 2.05, 2.87, and 3.94 meV. In addition, another broad inelastic scattering component extending from 0 to 1.5 meV has been found, which may also be important for the low-temperature behavior of the thermal conductivity. The line positions of the coupled modes shift to higher frequencies with increasing temperature, pointing towards the importance of the repulsive part of the host–guest interaction which is responsible for the stability of gas hydrate structures. © 2002 American Institute of Physics.
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  • 103
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    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 116 (2002), S. 3789-3794 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We studied the liquid–glass transition of SiO2 by means of replica theory, utilizing an effective pair potential which was proved to reproduce a few experimental features of silica. We found a finite critical temperature T0, where the system undergoes a phase transition related to replica symmetry breaking, in a region where experiments do not show any transition. The possible sources of this discrepancy are discussed. © 2002 American Institute of Physics.
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  • 104
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    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 116 (2002), S. 3805-3821 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The intermolecular nuclear magnetic resonance (NMR) chemical shifts of Xe in nanochannels of various dimensions and shapes are considered. Predictions of the line shapes that result from anisotropic averaging of the shielding tensor for Xe atoms in various limiting cases of nanochannels are made, based on ab initio calculations of 129Xe shielding surfaces. Variation of the line shapes with channel cross sections, Xe loading, and with temperature are predicted for channels having cross-sectional areas that do not permit two or more Xe centers to be located on the same cross-sectional plane. It is shown that Xe in effectively one-dimensional channels should be expected to exhibit signature line shapes in 129Xe NMR spectra, provided that the cross-sectional dimensions of the channels are sufficiently small that Xe atoms do not pass each other during diffusion. These predictions are tested against experimental examples of anisotropic Xe line shapes in various cavities and nanotubes. © 2002 American Institute of Physics.
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  • 105
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The molecular arrangement of monolayers and multilayers of cadmium behenate deposited at the air–water interface or on air-oxidized silicon wafers was investigated by means of grazing incidence x-ray diffraction. Even though it is frequently mentioned that the Langmuir–Blodgett (LB) monolayers of fatty acid salts have usually an in-plane hexagonal packing, we obtained two first-order peaks in all the investigated films, indicating a distorted hexagonal lattice. Whereas these peaks are in-plane in the Langmuir or LB monolayers implying no molecular tilt, they are slightly out of the plane in the 3 and 5 layers samples indicating tilted molecules. In these few layers LB films, second-order peaks are detected and the molecules are tilted towards a next-nearest neighbor (NNN structure), as in the L2′ phase of fatty acids. Using two angles of incidence of the x-ray beam allowing to probe either all the layers or only the external layers, a distribution of the tilt angle in the range 1°–6° is evidenced within each layer of the LB films. Moreover, the herringbone peak is measured in this NNN structure corresponding to the L2′ phase, implying that the herringbone order is long-range in the LB films. © 2002 American Institute of Physics.
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  • 106
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The pump power dependence of the relaxation dynamics of CdSe nanoparticles (NPs) was studied with femtosecond pump probe spectroscopy at observation wavelengths of the first exciton transition at 560 nm, the near infrared (NIR) absorption at 2 μm, and the transient mid-infrared (IR) absorption at 4.5 μm. Excitation with less than one photon per particle leads to bleaching of the excitonic transitions, and the bleach intensity is initially linear to the pump power. At higher pump power the bleach intensity levels off, when complete saturation of the excitonic transition is reached. At the same time, increasing pump power causes an acceleration of the bleach decay, which is due to additional Auger processes when multiple excitons are formed in the NPs. In addition, the pump power effect was investigated for the NIR and IR regions, at 2 and 4.5 μm wavelength, respectively. Whereas the IR transients are very similar to the ones observed for the bleach, the NIR transients behave completely different. No pump power dependence was found for the transients at 2 μm when pumped in a power range from 0.5 to 5 μJ per pulse. The results show that the fs transients in the visible (bleach) and IR (absorption) regions are due to electron relaxation in the conduction band and the NIR transients are due to the relaxation of the hole. Furthermore, it suggests that in the investigated CdSe NPs, Auger processes act much more efficiently on the electrons than for the holes. © 2002 American Institute of Physics.
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  • 107
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    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 116 (2002), S. 3834-3840 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We present a study on the ultrafast third-order nonlinearities of a novel series of iso-polydiacetylene oligomers (iso-PDAs). Unlike polydiacetylenes that contain a linearly-conjugated backbone, iso-PDAs have a backbone that is cross-conjugated. A new Kerr-gate technique, differential optical Kerr effect (DOKE) detection, is used to measure third-order nonlinear susceptibilities, χ(3), and second hyperpolarizabilities, γ, of monomer, dimer, trimer, pentamer, and heptamer samples in tetrahydrofuran (THF) solutions. A linear increase in γ as a function of the number of repeat units is observed for all samples except the monomer, suggesting that the fixed-length linearly conjugated segments dominate the electronic polarizability. An added increase to the oligomer nonlinearities due to communication along the cross-conjugated path is not observed. The largest nonlinearity was observed in the heptamer sample, displaying a second hyperpolarizability relative to the THF solvent of γheptamer/γTHF=181±9. In addition, an interesting feature arising out of the signal decay tail is present in the samples but absent in our THF solvent reference. © 2002 American Institute of Physics.
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  • 108
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    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 116 (2002), S. 3856-3863 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The energetics of nanoclusters is investigated for five different metals (Ag, Cu, Au, Pd, and Pt) by means of quenched molecular dynamics simulations. Results are obtained for two different semiempirical potentials. Three different structural motifs are considered: icosahedra (Ih), decahedra (Dh), and truncated octahedra (TO). The crossover sizes among structural motifs are directly calculated, considering cluster up to sizes N(similar, equals)40 000. For all the systems considered, it is found that icosahedra are favored at small sizes, decahedra at intermediate sizes, and truncated octahedra at large sizes. However, the crossover sizes depend strongly on the metal: in Cu, the icosahedral interval is rather large, and it is followed by a very wide decahedral window; on the contrary, in Au, the icosahedral interval is practically absent, and the decahedral window is narrow. The other metals display intermediate behaviors, Ag being close to Cu, and Pd and Pt being close to Au. A simple criterion, which is based on the ratio between the bulk modulus and the cohesive energy per atom, is developed to account for the differences among the metals. © 2002 American Institute of Physics.
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  • 109
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    The Journal of Chemical Physics 116 (2002), S. 3841-3855 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: By applying a corrugation-reducing procedure we have interpolated the six-dimensional (6D) potential energy surfaces for the H2/Pt(111) and H2/Cu(100) systems from data obtained by density functional theory (DFT) calculations. We have compared interpolated values with a large number of DFT results not used in the basis for the interpolation and we have obtained an average error below 20 meV and a maximum error of about 30 meV in the regions important for dissociative adsorption. Near the surface the corrugation-reducing procedure gives excellent results using only data from high-symmetry sites. However, we show that to reach the above mentioned accuracy level far from the surface, it is necessary to include information from at least one low-symmetry site. Care has been taken to demonstrate the quality of the interpolation along all degrees of freedom in different regions of the configuration space. The strengths of the method are shown together with the aspects requiring careful handling. A comparison with a direct interpolation method is also made. © 2002 American Institute of Physics.
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  • 110
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A model of the polarizability of carbon disulfide dimers was constructed, using polarizabilities from accurate time-dependent density functional theory calculations as reference. This direct reaction field model takes dipole-induced dipole effects, induced multipole effects and effects due to the overlap of the electronic clouds into account in an approximate way. The importance of the induced multipole and the overlap effects is investigated. This polarizability model is subsequently used to calculate the third-order time-domain Raman response of liquid carbon disulfide. These results are compared to experimental data and earlier calculated response in which only dipole-induced dipole effects on the polarizability were included. The multipole effects are found to give a significant contribution to the subpico second part of the third-order Raman response. © 2002 American Institute of Physics.
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  • 111
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    The Journal of Chemical Physics 117 (2002), S. 2435-2448 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Broadband dielectric spectroscopy, heat capacity spectroscopy (3ω method), and viscosimetry have been used to study the dynamic glass transition of two glass-forming epoxy resins, poly [(phenyl glycidyl ether)-co-formaldehyde] and diglycidyl ether of bisphenol-A. In spite of their rather simple molecular structure, the dynamics of these systems is characterized by two well-separated crossover regions where the relaxation times of main transition and the two secondary relaxations β and γ approach each other. The main transition has three parts: The a process at high temperature, the a′ process between the two crossover regions, and the α process at low temperatures. Both the γ-crossover region [around a temperature Tc(γ)∼(1.4–1.5)Tg and a relaxation time τc(γ)(approximate)10−10 s] and the β-crossover region [around Tc(β)∼(1.1–1.2)Tg and τc(β)(approximate)10−6 s] could be studied within the experimentally accessible frequency–temperature window. Different typical crossover properties are observed in the two regions. The γ-crossover region is characterized by onset of the (a′,α) process, with a relaxation time about one decade greater than that of the quasicontinuous (a,γ) trace. The β-crossover region is characterized, besides splitting of main andβ relaxation times, by a change in the temperature dependence of the main-relaxation time as reflected by a bend in the Stickel plot of the continuous (a′,α) trace, the separation of individual temperature dependences of different transport properties such as impurity-ions diffusion coefficient and viscosity, and a temperature-dependent main relaxation time that starts to be in accordance (at lower temperatures) with the Adam–Gibbs model. The cooperativity of the main process between the γ and β crossover seems to be small. Below the β crossover, cooperativity increases up to values of order Nα∼100 near Tg, and configurational entropy seems to correlate with the main relaxation time. © 2002 American Institute of Physics.
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  • 112
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    The Journal of Chemical Physics 117 (2002), S. 2449-2450 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The virial expression of the pressure of systems with periodic boundary conditions is discussed for long range Coulomb interactions. It is shown, that the mechanical definition of the pressure leading to the virial pressure agrees with the thermodynamical definition for such systems. © 2002 American Institute of Physics.
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  • 113
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: © 2002 American Institute of Physics.
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  • 114
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    The Journal of Chemical Physics 117 (2002), S. 1434-1440 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Fixed node diffusion Monte Carlo (FN-DMC) atomization energies are calculated for a benchmark set of 55 molecules. Using single determinant trial wave functions, comparison with experiment yields an average absolute deviation of 2.9 kcal/mol, placing this simplest form of FN-DMC roughly at the same level of accuracy as the CCSD(T)/aug-cc-pVQZ method. However, unlike perturbative wave function expansion approaches, FN-DMC is applicable to systems containing thousands of valence electrons. For the P2 molecule, a number of possible sources of error are explored in detail. Results show that the main error is due to the fixed-node approximation and that this can be improved significantly with multireference trial wave functions.
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  • 115
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    The Journal of Chemical Physics 117 (2002), S. 1441-1449 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We have investigated the performance of the new optimized exchange functional (OPTX) developed by Handy and Cohen [Mol. Phys. 99, 403 (2001)] for predicting geometries, heats of reaction, and barrier heights for twelve organic reactions (six closed-shell and six radical). OPTX has been used in conjunction with, among others, the well-known Lee–Yang–Parr (LYP) correlational functional to form two new functionals, OLYP and O3LYP. These are similar to the well-established BLYP and B3LYP functionals, respectively, with OPTX replacing the standard Becke exchange functional, B88. Our results strongly support claims made by their developers that OLYP is superior to BLYP, and essentially renders it obsolete. The computed OLYP heats of reaction, barrier heights, and even molecular geometries (with larger basis sets), are comparable with, if not better than, the corresponding B3LYP values. The O3LYP functional is overall better than B3LYP, albeit not by much. Both OLYP and O3LYP are among the best functionals currently available; the performance of OLYP in particular is noteworthy given that this functional includes no exact exchange. © 2002 American Institute of Physics.
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  • 116
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    The Journal of Chemical Physics 117 (2002), S. 1457-1469 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: General recurrence formulas for evaluating molecular integrals over contracted Cartesian Gaussian functions are derived by introducing auxiliary contracted hyper-Gaussian (ACH) functions. By using a contracted Gaussian function, this ACH represents an extension of the Gaussian function named derivative of Fourier-kernel multiplied Gaussian [J. Chem. Phys. 94, 3790 (1991)]. The ACH is reducible to contracted Cartesian Gaussian functions, contracted modified Hermite Gaussian functions, and to contracted Gaussian functions multiplied by phase factors, or the so-called GIAO, and is also reducible to various spatial operators necessary for ab initio molecular orbital calculations. In our formulation, all molecular integrals are expressed in terms of ACH. Therefore, the formulations have wide applicability for calculating various kinds of molecular integrals in ab initio calculations. Recursive calculations based on our formulation do not depend on the number of contraction terms, because the contraction step is completed at the evaluation of the initial integrals. Therefore, we expect that more efficient recursive calculations will be accomplished by using our formulas for evaluating molecular integrals over contracted Gaussian functions. © 2002 American Institute of Physics.
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  • 117
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    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Variational transition state theory with multidimensional tunneling contributions (VTST/MT) is tested against quantum mechanical rate constants for the reaction H+CH4→H2+CH3 at temperatures up to 1000 K. The VTST/MT method can be and has been applied to many reactions that cannot be treated by rigorous quantum dynamics methods. Studying the accuracy of VTST/MT by comparison with accurate quantal results that are becoming available for systems of increasing size is important for validating the theory. In the present study, covering a factor of five in temperature, the VTST/MT method is found to have a mean deviation from accurate quantal rate constants for a six-body reaction of only 13% and maximum deviation of only 23%. © 2002 American Institute of Physics.
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  • 118
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    The Journal of Chemical Physics 116 (2002), S. 3396-3400 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A continuum elastic model of fullerenes is presented by utilizing the analogy between the closed carbon cages and elastic shells. We derive expressions for the curvature related strain energies Ep of the pentagonal protrusions. We propose to explain the observed sphericity of the carbon onions shells as opposed to the predicted protrusions around the pentagonal defects on the basis of our continuum elastic model of fullerenes. In our model the energy inherent in the pentagonal protrusions Ep is due to the stretching and bending of the shell and shown to be a function of the structural parameters. It also defines the upper limit on the size of the free-standing fullerenes. Using Ep and the topological arguments, we show that the pentagonal protrusions will be smoothed out, resulting in spherical shells of the carbon onions denoted as C60@C240@C540@C960@C1500,... . © 2002 American Institute of Physics.
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  • 119
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    The Journal of Chemical Physics 116 (2002), S. 3420-3429 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Vibronic interaction and its role in the occurrence of possible superconductivity in the monoanions of phenanthrene-edge-type aromatic hydrocarbons are studied. The vibrational frequencies and the vibronic coupling constants are computed and analyzed and the electron–phonon coupling constants are estimated. The results for phenanthrene-edge-type hydrocarbons are compared with those for acene-edge-type hydrocarbons. The lowest frequency mode and the C–C stretching modes of 1400–1600 cm−1 afford large electron–phonon coupling constants in the monoanions of acene- and phenanthrene-edge-type hydrocarbons. The total electron–phonon coupling constants decrease with an increase in the number of carbon atoms in both acene- and phenanthrene-edge-type hydrocarbons, but those for the monoanions of phenanthrene-edge-type hydrocarbons are larger than those for the monoanions of acene-edge-type hydrocarbons. Possible superconducting transition temperatures Tcs for the monoanions are estimated. The monoanions of phenanthrene-edge-type hydrocarbons would have higher Tcs than the monoanions of acene-edge- type hydrocarbons if phenanthrene-edge-type hydrocarbons exhibit superconductivity. These results suggest that molecular edge structures as well as molecular sizes have relevance to the strength of electron–phonon coupling and Tcs. The fragment molecular-orbital method (FMO) method successfully characterizes the distinct electronic structures of the two small polynuclear aromatic hydrocarbons (PAHs) with different type of edges such as anthracene and phenanthrene. © 2002 American Institute of Physics.
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  • 120
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A computationally efficient method was developed for calculating Coulomb interactions in three-dimensional (3D) systems with two-dimensional (2D) periodicity; the 2D particle-mesh Ewald (2D-PME) method we previously developed was extended. The formulation and numerical algorithms are described in detail for calculating the Coulomb potential energy, the Coulomb force, and the Coulomb component of the pressure tensor. Computational efficiency and accuracy of the 2D-PME method were evaluated for two water systems with 2D periodicity in the x and y directions and with non-periodicity in the z direction. Compared with exact results calculated by using the original 2D Ewald summations, the 2D-PME method yielded significantly accurate calculations, similar to the computationally efficient method we previously developed for calculating 2D Ewald summations (2D-EW method). For a given accuracy, the 2D-PME method was faster than the 2D-EW method for the water systems we examined. The computational effort of the 2D-PME method decreases as the computationally efficiency of the Fourier transforms used in the 2D-PME method increases. The 2D-PME method is therefore promising for accelerating molecular dynamics and Monte Carlo simulations for 3D systems with 2D periodicity. © 2002 American Institute of Physics.
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  • 121
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    The Journal of Chemical Physics 116 (2002), S. 3471-3483 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Electrostatically stabilized aqueous dispersions of nm-sized free lipid A particles at low volume fractions (1.0×10−4≤&nullset;≤3.5×10−4) in the presence of 1.0–10.0 mM NaCl (25 °C) have been characterized by static and quasielastic light scattering (QELS) techniques, electron microscopy (SEM and TEM), conductivity measurements, and acid–base titrations. QELS and electron microscopy (ρ¯TEM=8.0±0.6%) yield similar values for the particle size and particle size distribution (ρ¯QELS=10.9±0.75 %), whereas conductivity and acid–base titrations estimate surface chemical parameters (dissociation constant, ionizable sites, and Stern capacitance). Effective particle charges were determined by fits of the integral equation calculations of the polydisperse static structure factor, S¯(Q), to the light scattering data. Using the particle properties as determined from these experiments, the polydisperse structure factor, S¯(Q), was calculated as a function of volume fraction, &nullset;, which was found to be consistent with a S¯(Q) dependence on the number particle density. It can be concluded that, at low volume fractions and low ionic strength, the light scattering data are well represented by a Poisson–Boltzmann model (PBC) of fluid-like ordering of free lipid A in aqueous solution. We find that the light scattering data of this dispersion are best described by a model where only a small fraction of the ionizable phosphate groups is dissociated at neutral pH. Finally, light scattering studies of lipid A dispersions of volume fractions of 3.9×10−4≤&nullset;≤4.9×10−4 indicate the presence of long-range order, resulting in distinct peaks which can be assigned either to a face-centered cubic (fcc) lattice (a=51.7 nm) or a body-centered cubic (bcc) lattice (a=41.5 nm), respectively. © 2002 American Institute of Physics.
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  • 122
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    The Journal of Chemical Physics 116 (2002), S. 3484-3492 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We investigate vapor–liquid equilibria of dendrimer/solvent (benzyl ether dendrimer/toluene) systems by the combination of incompressible lattice cluster theory and atomistic simulation technique. We also examine the structure effect of dendritic polymer and the specific interaction due to the difference of interaction energies of endgroup at the periphery of dendrimer molecules. The interaction energy parameters are obtained by the pairs method including Monte Carlo simulation technique with excluded volume constraint. In the pairs method, we do not simulate the whole molecule as in molecular dynamics or molecular mechanics, but only monomer segments interacting with solvent molecules. In general, those parameters are determined by fitting experimental data. Our results show that the specific interactions between the endgroup and the solvent molecule play an important role in determining phase behaviors of the given systems. © 2002 American Institute of Physics.
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  • 123
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The reaction of ground-state atomic oxygen (O(3P)) with allyl radical (C3H5) was investigated in the crossed beam configuration. O(3P) and C3H5 were generated by the photodissociation of NO2 and the supersonic flash pyrolysis of allyl iodide, respectively. The nascent internal distributions of the OH(X2Π : v″=0,1) reaction product from the newly observed channel of O(3P)+C3H5→C3H4+OH were probed by laser induced fluorescence (LIF) spectroscopy. The distributions showed significant excitations with an unusual bimodal feature: the low and high rotational components without spin-orbit and Λ-doublet propensities in the ground and first excited vibrational states. On the basis of population analysis and comparison with the ab initio and statistical calculations, the experimental distributions are estimated to be totally non-statistical and suggest that the dynamics of the reaction might be described by two competing mechanisms: a major direct abstraction process and an indirect short-lived addition-complex forming process. © 2002 American Institute of Physics.
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  • 124
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    The Journal of Chemical Physics 116 (2002), S. 3502-3502 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: © 2002 American Institute of Physics.
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  • 125
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    The Journal of Chemical Physics 116 (2002), S. 2684-2687 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Using the technique of Cs+ reactive ion scattering (Cs+ RIS), we show that reaction of CO and O on a Pt(111) surface produces a CO2 precursor species that remains stable on the surface up to 300 K. The CO2 species desorbs with an activation energy of 46±3 kJ mol−1, whereas the direct CO2 gas formation via Langmuir–Hinshelwood (LH) reaction between CO and O occurs with an activation energy of 75±4 kJ mol−1. The observation reveals a new pathway for CO oxidation reaction, which is different from the direct LH pathway. © 2002 American Institute of Physics.
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  • 126
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    The Journal of Chemical Physics 116 (2002), S. 2688-2691 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We give a lower bound on the diffusion coefficient of a polymer chain in an entanglement network with kinematic disorder, which is obtained from an exact calculation in a modified Rubinstein–Duke lattice gas model with periodic boundary conditions. In the limit of infinite chain length we show the diffusive motion of the polymer to be slowed down by kinematic disorder by the same factor as for a single particle in a random barrier model. © 2002 American Institute of Physics.
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  • 127
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    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The quantum dynamics of the HF molecule is investigated in the presence of a static electric field and coherent infrared radiation. The time dependent dynamics is induced by infrared multiphoton excitation and the time evolution of the rovibrational wave packet is calculated in configuration space. Results are given for static field strengths needed to orient molecules such that their internal dynamics can be described by simplified, purely vibrational models. For highly oriented HF molecules, time intervals of approximate duration of 70 fs occur repeatedly during and after the excitation in which the reduced vibrational wave packet motion is nearly semiclassical ("semiclassical windows"). The occurrence of these time intervals can be made more regular after the excitation, if the pulse duration is chosen adequately. © 2002 American Institute of Physics.
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  • 128
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    The Journal of Chemical Physics 116 (2002), S. 2728-2747 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Accurate estimates of the magnetic coupling in binuclear complexes can be obtained from ab initio configuration interaction (CI) calculations using the difference dedicated CI technique. The present paper shows that the same technique also provides a way to analyze the various physical contributions to the coupling and performs numerical analysis of their respective roles on four binuclear complexes of Cu (d9) ions. The bare valence-only description (including direct and kinetic exchange) does not result in meaningful values. The spin-polarization phenomenon cannot be neglected, its sign and amplitude depend on the system. The two leading dynamical correlation effects have an antiferromagnetic character. The first one goes through the dynamical polarization of the environment in the ionic valence bond forms (i.e., the M+(centered ellipsis)M− structures). The second one is due to the double excitations involving simultaneously single excitations between the bridging ligand and the magnetic orbitals and single excitations of the environment. This dispersive effect results in an increase of the effective hopping integral between the magnetic orbitals. Moreover, it is demonstrated to be responsible for the previously observed larger metal-ligand delocalization occurring in natural orbitals with respect to the Hartree–Fock ones. © 2002 American Institute of Physics.
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  • 129
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    The Journal of Chemical Physics 116 (2002), S. 2748-2756 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: It is shown that the accuracy of quantum dynamics calculations obtained according to the Herman–Kluk (HK) semiclassical initial value representation (SC-IVR) is significantly improved when the time evolution operator is computed by concatenating finite time propagators. This approach results in an approximate calculation of a real-time path-integral in a discrete coherent-state representation, which becomes exact in the limit of sufficiently short time-slice intervals. The efficiency of the computational method is optimized by devising a compact coherent-state basis set that obviates the need for calculating the inverse overlap matrix. Quantitative agreement with full quantum mechanical results is verified in the description of tunneling between disjoint classically allowed regions in one- and two-dimensional systems, in the treatment of long-time dynamics, and in nonadiabatic dynamics in a model system with two coupled one-dimensional potential energy surfaces. © 2002 American Institute of Physics.
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  • 130
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    The Journal of Chemical Physics 116 (2002), S. 2763-2775 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The primary photodissociation dynamics of allyl chloride upon excitation at 193 nm is investigated in a crossed laser-molecular beam scattering apparatus. Tunable vacuum ultraviolet (VUV) photoionization of the products provides a unique ability to learn about the secondary reaction products of the nascent photoproducts formed. The data show evidence for four significant primary reaction channels: a previously unidentified low kinetic energy C–Cl bond fission channel producing unstable allyl radicals, an excited state C–Cl bond fission channel producing Cl atoms with high translational energy, an HCl elimination pathway releasing significant energy to product translation to HCl and its momentum-matched mass 40 partner, and an HCl elimination channel producing low kinetic energy HCl products and predominantly unstable mass 40 products. The measured branching of these primary reaction channels of [all C–Cl] : [fast C–Cl] : [slow C–Cl] : [fast HCl] : [slow HCl] : [all HCl] is 1.00: 0.971: 0.029: 0.291: 0.167: 0.458 (where fast refers to the high recoil kinetic energy channels). The high internal energy allyl radicals formed in the slow C–Cl fission pathway of allyl chloride further dissociate/isomerize, as do the unstable mass 40 products formed in the HCl elimination pathways, and these products are investigated. Photoionization efficiency (PIE) curves of the HCl product suggest that a three-centered elimination mechanism contributes significantly to an observed HCl elimination reaction. © 2002 American Institute of Physics.
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  • 131
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    Topics: Physics , Chemistry and Pharmacology
    Notes: The photodissociation dynamics of COCl2 has been studied by monitoring ground Cl(2P3/2) and spin–orbit excited Cl*(2P1/2) fragments by applying a novel technique where the three-dimensional momentum vector of a single reaction product is directly determined. The photodissociation at 235 nm produces exclusively three fragments: COCl2+hν→CO+2Cl. The kinetic energy distributions of Cl and Cl* are bimodal and exhibit a different behavior for the different spin–orbit states. Our attention was turned to the dependence of the anisotropy parameter β on the fragment velocity which was observed for the first time. For both spin–orbit states the anisotropy parameter differs clearly for slow and fast chlorine atoms, where a pronounced change from the value ∼0.7 to zero at about 20 kJ/mol is observed. Slow chlorine atoms are released isotropically and predominantly in the ground state Cl whereas fast chlorine atoms have an anisotropy parameter close to the theoretically limiting value and are distributed between ground and excited state Cl. These observations can be explained by a sequential decay where the first Cl fragment is released in a fast process characterized by the nonvanishing positive β parameter and a lifetime of ≤210 fs, whereas the second Cl fragment is released after a period which is long on a rotational time scale. A significant contribution of a symmetric mechanism can be excluded. © 2002 American Institute of Physics.
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  • 132
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    Topics: Physics , Chemistry and Pharmacology
    Notes: When the spin–orbit interaction is included, the character of a conical intersection in a molecule with an odd number of electrons differs dramatically from that of its nonrelativistic counterpart. In contrast to the two-dimensional branching space (η=2) in the nonrelativistic case, for these conical intersections the branching space is five-dimensional (η=5) in general, or three-dimensional (η=3) when Cs symmetry is present. Recently we have introduced an algorithm, based on analytic gradient techniques, to locate such conical intersections and used related techniques to efficiently construct and study the properties of the vectors defining the branching space. Here we extend this analysis. A perturbative description of the η=3 case is reported and used to determine the energy, derivative couplings, and a "rigorous" diabatic basis in the vicinity of a conical intersection. The perturbative results are compared with those of exact numerical calculations employing model Hamiltonians. The implications for the nuclear motion problem are discussed. © 2002 American Institute of Physics.
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  • 133
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    The Journal of Chemical Physics 116 (2002), S. 2848-2851 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: It is known that SO42− is not electronically stable as an isolated species but can be rendered stable by solvation (e.g., by adding a few H2O molecules). Recently, our group introduced a Coulomb repulsion model that offers an approximation to the energy instability and lifetimes of such species. In order to achieve an independent and likely more reliable estimate of the instability of SO42−, we have undertaken a follow-up study of this dianion. Specifically, we apply a stabilization method to determine the vertical electronic energy difference between the metastable SO42− dianion and its SO4−1 daughter at several levels of theory. The particular variant of the stabilization method used here involves adding a partial positive charge to the central sulfur nucleus in order to confine the escaping electron. Our coupled-cluster data, which represent our highest level of theory, suggest that SO42− is unstable by 1.1 eV and has a lifetime with respect to electron loss of 1.6×10−10 s (our earlier estimates were 0.75 eV and 2.7×10−8 s). © 2002 American Institute of Physics.
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  • 134
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    The Journal of Chemical Physics 116 (2002), S. 2565-2571 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Water may be adsorbed in microporous or mesoporous materials from the liquid phase or from the gaseous, or vapor, phase. In either case, the fluid state of the adsorbed water molecules may be liquid or vapor as well, depending on several factors inherent to the adsorbate itself. This work sought to understand these factors using an equation of state reported in the literature recently. It was determined that the model's predicted contribution of the hydrogen bonding to the Helmholtz free energy was less than expected. The estimation of the magnitude of hydrogen bonding appears to be more realistic if positive contributions of four neighbors is included rather than one. The model also appears to involve quite sensitive calculations, which may be prone to precision errors. © 2002 American Institute of Physics.
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  • 135
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    Topics: Physics , Chemistry and Pharmacology
    Notes: Protonated ammonia clusters of the composition (NxH3x+1)+ with x=2,3,4 are investigated by using the gradient corrected, three-parameter functional by Becke based on the functional by Lee, Yang, and Parr (B3LYP/6-31G**) and self-consistent charges density functional tight-binding (SCC–DFTB) methods for calculating the potential energy surface and forces in the Born–Oppenheimer approximation. They are used for classical molecular dynamics simulations at temperatures ranging from 5 K to 600 K. Results from the two methods are compared for proton transfer in N2H7+. The number of proton transfer events as a function of temperature is similar, although at low temperatures, SCC–DFTB cuts off more rapidly than B3LYP/6-31G**. Calculated vibrational spectra agree well for the intermolecular N–N and intramolecular N–H stretch excitations. Both approaches lead to broad, relatively unstructured bands extending over about 1500 cm−1 for the proton transfer coordinate. Simulations at the SCC–DFTB/MD level for larger (NxH3x+1)+ (x≤4) clusters are presented and discussed. They show significant structural reorganization within the cluster. Consecutive proton hops within a few tenths of a fs are observed. A N2H7+ cluster immersed in a water shell containing 25 water molecules was studied by the mixed quantum mechanical/molecular mechanics (QM/MM) method with SCC–DFTB for the QM part. The presence of water appears to impede proton transfer. Including corrections for basis set superposition error in the MP2/aug-cc-pVTZ and B3LYP/6-31G** calculations has a small effect. It increases the barrier heights from 0.78 kcal/mol to 1.28 kcal/mol (MP2) and from 0.10 kcal/mol to 0.27 kcal/mol (B3LYP), respectively. © 2002 American Institute of Physics.
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  • 136
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    The Journal of Chemical Physics 116 (2002), S. 2609-2617 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Cancrinite crystals possessing parallel nanochannels are attractive for incorporation of guest materials and preparation of one-dimensional structures. In this work, we study variety of cancrinite crystals synthesized with Se inside their channels. Single crystal x-ray diffraction, polarized Raman, optical absorption, and luminescence spectra are investigated. It is shown that Se is stabilized in the form of Se22− and Se2− dimers located in the center of the channel and oriented along the channel. Different absolute and relative concentrations of Se22− and Se2− are obtained for different samples. The Se22− dimers at high concentration show tendency to organize linear chains. At low temperatures, quite strong interdimer bonding for both Se22− and Se2− is observed. Another important low-temperature effect is appearance of additional Raman bands, which are attributed to the vibrations of linear Se22− chains distorted by the incommensurate potential of cancrinite. Strong near-infrared polarized luminescence is observed for all samples. Photoionization of dimers is shown to be important step in the mechanism of the luminescence. © 2002 American Institute of Physics.
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  • 137
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    The Journal of Chemical Physics 116 (2002), S. 2599-2608 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A simple kinetic model is used to describe the interaction of H and D atomic beams with H- and D-covered metal surfaces. The atoms incident from the gas phase can have a direct Eley–Rideal reaction with an adsorbate, reflect, penetrate into the bulk, knock an adsorbate out of its binding site, or trap to form a hot atom. These hot mobile atoms can go on to react with other adsorbates, or eventually relax and stick. A coarse-graining approach, which takes advantage of the large difference between the time scales for the kinetics experiments and the reaction dynamics, allows us to derive relatively simple kinetic equations for reaction rates and coverages. The approach is similar to a kinetic random walk model developed by Küppers and co-workers [J. Phys. Chem. 109, 4071 (1998)] except that our equations can be used to derive analytical expressions for saturation coverages, rates, and yields. The model is applied to the case of H atom reactions on a Ni(100) surface, and a detailed comparison is made with both experimental and quasiclassical studies. © 2002 American Institute of Physics.
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  • 138
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    The Journal of Chemical Physics 116 (2002), S. 2592-2598 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We study the spectral response of the transition between the first and the second excited state of the O–H stretch vibration of HDO dissolved in liquid D2O with two-color femtosecond mid-infrared spectroscopy. The spectral response of this transition differs strongly from the fundamental absorption spectrum of the O–H stretch vibration. In addition, excitation of the O–H stretch vibration is observed to lead to a change of the hydrogen-bond dynamics of liquid water. We show that both these observations can be described with a refined quantum-mechanical version of the Lippincott–Schroeder model for hydrogen-bonded OH(centered ellipsis)O systems. © 2002 American Institute of Physics.
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  • 139
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    The Journal of Chemical Physics 116 (2002), S. 2586-2591 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: From a variety of long simulations of all-atom butane using both stochastic and fully solved molecular dynamics, we have uncovered striking generic behavior that also occurs in one-dimensional systems. We find an apparently universal distribution of transition event durations, as well as a characteristic speed profile along the reaction coordinate. An approximate analytic distribution of event durations, derived from a one-dimensional model, correctly predicts the asymptotic behavior of the universal distribution for both short and long durations. © 2002 American Institute of Physics.
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  • 140
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    The Journal of Chemical Physics 116 (2002), S. 2618-2627 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A parametrization algorithm to determine chord-length distributions (CLD) from small-angle scattering data is used to characterize details of the pore structure, pore size, and morphology of porous carbon materials. This approach is applied to various carbon processing procedures, such as steam activation, tempering, and contacting with polymer binders. It is found that this CLD method reveals, compared to previous small angle x-ray scattering evaluation approaches, fine changes in the pore structure and morphology of porous carbons with higher accuracy and physical significance. © 2002 American Institute of Physics.
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  • 141
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    The Journal of Chemical Physics 116 (2002), S. 2628-2635 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We present a theoretical study of several hydrogen defects in the upper mantle mineral forsterite using a quantum mechanical embedded cluster approach. We make extensive comparisons with results obtained using Mott–Littleton calculations and data from periodic density functional studies by ourselves and other authors. We show that the embedded cluster method gives reliable energies for a range of defect reactions, including cation vacancy formation, binding between defects, and formation of hydrogen defects by reaction with water. We propose that the embedded cluster techniques should be the method of choice when studying point defects in minerals. © 2002 American Institute of Physics.
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  • 142
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    The Journal of Chemical Physics 116 (2002), S. 2636-2641 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The mechanical and dielectric relaxation of polymer networks depends (especially in simple Gaussian-type approaches which extend the Rouse model) on the eigenvalues of the corresponding connectivity matrices. We use this to evaluate explicitly experimentally accessible relaxation forms for finite Sierpinski-type networks, whose eigenvalue spectra are multifractal. It turns out that the observable quantities are by far less singular than the eigenvalue spectra, since the underlying spectral structures get smoothed out. Our results establish unequivocally the spectral dimension as fundamental relaxation parameter; to see this, however, the finite fractal networks have to be sufficiently large. © 2002 American Institute of Physics.
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  • 143
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    The Journal of Chemical Physics 116 (2002), S. 2642-2649 
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    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The collapse of Lennard-Jones homopolymers is investigated by means of Monte Carlo simulations and the inherent structure/superposition approach, with special emphasis on finite size effects. At thermal equilibrium, the polymers undergo a series of phase changes from the zero temperature folded state to a coexistence state, a molten globule state, the coil state, and finally to a high-temperature "soft" state where the bond lengths vary significantly from their equilibrium value. The correlation between the thermodynamic characteristics of the polymers and those of clusters is interpreted in terms of the energy landscapes of the two systems. © 2002 American Institute of Physics.
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  • 144
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    Topics: Physics , Chemistry and Pharmacology
    Notes: A model system of charged hard spheres and point counterions is used to analyze a de-ionized colloidal dispersion. Thermodynamic properties are calculated using a symmetric Poisson–Boltzmann equation, a linear form, and the Mean Spherical Approximation theory. The theories are found to predict a gas–liquid type transition and the spinodals computed. The results are compared among themselves and with recent Monte Carlo simulations for the model. © 2002 American Institute of Physics.
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  • 145
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    The Journal of Chemical Physics 116 (2002), S. 2658-2662 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Although rheology as well as neutron and x-ray scattering studies of ordered dispersions have been performed no coherent picture is discussable. In this paper we report our synchrotron x-ray and neutron scattering data of concentrated, charge stabilized, monodisperse polymer colloids and compare them with rheological data. We found that the scattering data can be understood in terms of viscoelastic flow. When ordered by flow-, solid- and liquid-like behavior can be separated with two independent experiments: (a) The solid-like character can be seen at perpendicular incidence as Bragg reflections, (b) the liquid-like character becomes apparent by sample rotation as the intensity distribution along Bragg rods. © 2002 American Institute of Physics.
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  • 146
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    The Journal of Chemical Physics 116 (2002), S. 2663-2664 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A series of classical molecular dynamics simulations of a water wire in a DPPC bilayer yielded lifetimes τ of up to 90 ps, with τ¯=37±8 ps (not including a significant fraction wires that broke during equilibration). This is sufficiently long to conduct protons across the bilayer through a quantum mechanical hopping mechanism, similar to proton conduction in ice. Lifetimes of wires in the octane region of a water/octane/water "sandwich" were qualitatively similar (τ¯=36±3 ps) though wires were rarely lost during equilibration. Hence, the utility of the water/octane system as a membrane surrogate depends on the application.
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  • 147
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    Topics: Physics , Chemistry and Pharmacology
    Notes: An accurate, physically justifiable procedure for determining zero-PMF baselines is essential in addressing the important issue of anti-cooperativity or cooperativity in hydrophobic interactions. For this purpose, the test-particle insertion technique is less dependent on unproven assumptions than other procedures. Its application is recommended for studies in which the technique is computationally feasible. We agree with Czaplewski et al. that much remains to be learned about hydrophobic interactions nonadditivity, and that sustained efforts are needed to overcome numerical uncertainties in the simulation results reported thus far. Nonetheless, with data available to date, there is apparently stronger support for anti-cooperativity than for cooperativity for a significant fraction of solute configurations of the three-methane system in question. This is illustrated by a reanalysis of a three-methane PMF reported previously by Czaplewski et al. [Protein Sci. 9, 1235 (2000)] in light of the new two-methane PMF presented in their Comment. © 2002 American Institute of Physics.
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  • 148
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    Topics: Physics , Chemistry and Pharmacology
    Notes: Inclusion complex formation between a conducting polymer, polyaniline (PANI) with emeraldine base, and a molecular nanotube synthesized from α-cyclodextrin (α-CD) has been studied by atomic force microscopy. We observed a rodlike inclusion complex of PANI and the molecular nanotube on mica substrate at room temperature. The height of this structure is nearly equal to the outside diameter of α-CD and almost uniform along the whole length of the structure, which indicates that a conducting wire of PANI is fully covered by molecular nanotubes as insulator. Accordingly, this inclusion complex can be regarded as insulated molecular wire. © 2002 American Institute of Physics.
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  • 149
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    The Journal of Chemical Physics 116 (2002), S. 1757-1759 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Wertheim's first order thermodynamic perturbation theory [M. S. Wertheim, J. Chem. Phys. 87, 7323 (1987)] has been extended to the two-dimensional tangent hard disk dimer for both the solid and the fluid phases. This extension utilizes pre-existing equations of state for the fluid [C. F. Tejero and J. A. Cuesta, Phys. Rev. E 47, 490 (1993)] and for the solid phases [B. J. Alder, W. G. Hoover, and D. A. Young, J. Chem. Phys. 49, 3688 (1968)], of the two-dimensional hard disk monomer system. The theory has been compared to simulation results and a good agreement has been found between them. Results are also presented for the fluid–solid tie line for m=2 up to m=30. The results of this work may be of interest to those involved in the study of chainlike systems adsorbed on a two-dimensional surface. © 2002 American Institute of Physics.
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  • 150
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    Topics: Physics , Chemistry and Pharmacology
    Notes: We address the criticism of our methodology for determination of the three-body cooperative terms in the potential of mean force (PMF) of the hydrophobic interaction of methane molecules in water [Czaplewski et al., Prot. Sci. 9, 1235 (2000)] expressed in the title paper of Shimizu and Chan, as well as their conclusion that hydrophobic association is predominantly anti-cooperative. We demonstrate that their reference two-methane PMF curve is subject to a systematic error, which invalidates their conclusions about the sign of the cooperative PMF. © 2002 American Institute of Physics.
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  • 151
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    The Journal of Chemical Physics 116 (2002), S. 1760-1772 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: Theoretical analysis and results of calculations are put forward to interpret the energies −εk of the occupied Kohn–Sham (KS) orbitals as approximate but rather accurate relaxed vertical ionization potentials (VIPs) Ik. Exact relations between εk and Ik are established with a set of linear equations for the εk, which are expressed through Ik and the matrix elements εkresp of a component of the KS exchange-correlation (xc) potential vxc, the response potential vresp. Although −Ik will be a leading contribution to εk, other Ij≠k do enter through coupling terms which are determined by the overlaps between the densities of the KS orbitals as well as by overlaps between the KS and Dyson orbital densities. The orbital energies obtained with "exact" KS potentials are compared with the experimental VIPs of the molecules N2, CO, HF, and H2O. Very good agreement between the accurate −εk of the outer valence KS orbitals and the corresponding VIPs is established. The average difference, approaching 0.1 eV, is about an order of magnitude smaller than for HF orbital energies. The lower valence KS levels are a few eV higher than the corresponding −Ik, and the core levels some 20 eV, in agreement with the theoretically deduced upshift of the KS levels compared to −Ik by the response potential matrix elements. Calculations of 64 molecules are performed with the approximate vxc obtained with the statistical averaging of (model) orbitals potentials (SAOP) and the calculated εk are compared with 406 experimental VIPs. Reasonable agreement between the SAOP −εk and the outer valence VIPs is found with an average deviation of about 0.4 eV. © 2002 American Institute of Physics.
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  • 152
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    The Journal of Chemical Physics 116 (2002), S. 1773-1782 
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    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The first implementation of analytic gradients for the coupled-cluster singles, doubles, triples (CCSDT) model is described. The relevant theoretical expressions are given in a diagrammatic form together with the corresponding algebraic formulas. The computational requirements of CCSDT gradient calculations are discussed and their applicability demonstrated by performing benchmark calculations for molecular geometries with large correlation-consistent basis sets. A statistical analysis of the data reveals that CCSDT and CCSD(T) in most cases perform equally well. The CCSDT calculations thus provide further evidence for the high accuracy of the CCSD(T) approach. © 2002 American Institute of Physics.
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  • 153
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    The Journal of Chemical Physics 116 (2002), S. 1783-1787 
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    Topics: Physics , Chemistry and Pharmacology
    Notes: A new Monte Carlo move for polymer simulations is presented. The "wormhole" move is build out of reptation steps and allows a polymer to reptate through a hole in space; it is able to completely displace a polymer in time N2 (with N the polymer length) even at high density. This move can be used in a similar way as configurational bias; in particular, it allows grand canonical moves, it is applicable to copolymers, and can be extended to branched polymers. The main advantage is speed, since it is exponentially faster in N than configurational bias, but is also easier to program. © 2002 American Institute of Physics.
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  • 154
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    The Journal of Chemical Physics 116 (2002), S. 1800-1810 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We present a modified version of the polarized atomic orbital (PAO) method [M. S. Lee and M. Head-Gordon, J. Chem. Phys. 107, 9085 (1997)] to construct minimal basis sets optimized in the molecular environment. The minimal basis set derives its flexibility from the fact that it is formed as a linear combination of a larger set of atomic orbitals. This approach significantly reduces the number of independent variables to be determined during a calculation, while retaining most of the essential chemistry resulting from the admixture of higher angular momentum functions. Furthermore, we combine the PAO method with linear scaling algorithms. We use the Chebyshev polynomial expansion method, the conjugate gradient density matrix search, and the canonical purification of the density matrix. The combined scheme overcomes one of the major drawbacks of standard approaches for large nonorthogonal basis sets, namely numerical instabilities resulting from ill-conditioned overlap matrices. We find that the condition number of the PAO overlap matrix is independent from the condition number of the underlying extended basis set, and consequently no numerical instabilities are encountered. Various applications are shown to confirm this conclusion and to compare the performance of the PAO method with extended basis-set calculations. © 2002 American Institute of Physics.
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    The Journal of Chemical Physics 116 (2002), S. 1788-1799 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: In a recently reported method, the molecular density is partitioned in minimally deformed atomic contributions, which are expanded in spherical harmonics times radial factors. Here we use this representation to express the electrostatic potential of the molecule, the force on its nuclei, and the conformational variations of energy in terms of some simple integrals of the atomic radial factors. As a first application, we analyze the relationship between the density and the binding forces (and the bonding energy) in the diatomic molecules of the first row atoms. Two types of forces act on each nucleus: the self-pulling exerted by its own cloud and the external force due to the remaining atoms. The self-pulling comes only from the dipole type term of the atomic density. The external force comes from the other clouds and nuclei and is dominated by the effective charges which depend on the outermost region of the charge term. Analyzing the progressive deformations of the atoms when they approach each other, the forces associated with these deformations and their contributions to the energy, one has a detailed description of the chemical bond which is complementary, and in many aspects more appealing, than the conventional ones. © 2002 American Institute of Physics.
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  • 156
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    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: In a previous paper of this series [Paper III: Nakatsuji, J. Chem. Phys. 105, 2465 (2001)], the author showed a high potentiality of the extended coupled cluster (ECC) method to calculate the exact wave function of the ground state. In this paper, we propose ECC-configuration interaction (CI) method, which is an accurate useful method to calculate the excited states from the ECC wave function of the ground state. In contrast to the ECC method, the standard ECC-CI method is approximate, but we can make it exact by generalizing its excitation operator (ECC-CI general). The ECC-CI method is applicable not only to the excited states having the same spin-space symmetry as the ground state, but also to those having different spin-space symmetries and to the ionized and electron-attached states. The theoretical framework of the ECC-CI method is similar to that of the symmetry-adapted-cluster (SAC)-CI method proposed in 1978 by the present author. Next in this paper, we examine the performance of the methods proposed in this series of papers for a simple one-dimensional harmonic oscillator. The iterative configuration interaction (ICI) and ECC methods are examined for the ground state and the ICI-CI and ECC-CI methods for the excited states. The ICI method converges well to the exact ground state and the excited states are calculated nicely by the ICI-CI method in both the standard and general active spaces. In contrast to the simplest (S)ECC examined in Paper III, the ECC2 method shows quite a rapid convergence to the exact ground state, which enables us to calculate the true exact wave function in the ECC form. The ECC-CI methods in both the standard and general active spaces also work well to calculate the excited states. Thus, we conclude that the ICI and ECC approaches have a potentiality to provide useful method to calculate accurate wave functions of the ground and excited states. A merit of ECC is that it provides the exact wave function in a simple explicit form. © 2002 American Institute of Physics.
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  • 157
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    The Journal of Chemical Physics 116 (2002), S. 1834-1838 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The dynamical behavior and equilibrium distribution of linear bead-rod and bead-spring polymers differ even in the limit of infinitely stiff springs. Imposing metric pseudopotential forces on the bead-rod chains yields the behavior of bead-spring chains in Langevin and Brownian Dynamics simulations. Here we present a simple, compact, and efficient algorithm for computing the required metric correction forces at minimal computational cost. © 2002 American Institute of Physics.
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  • 158
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    The Journal of Chemical Physics 116 (2002), S. 1825-1833 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: In this work, geometric algebra is applied to obtain the volume-element of integration for the 3 Cartesian coordinates of the center-of-mass, 3 Euler angles, and 3N−6 shape coordinates needed to describe the position, orientation, and shape of an N-atomic molecule. The volume-element is obtained as a product of N volume-elements, each associated with a set of three coordinates. The method presented has several advantages. For example, one does not need to expand any determinants, and all calculations are performed in the three-dimensional physical space (not in some 3N-dimensional abstract configuration space). Several examples and applications are given. © 2002 American Institute of Physics.
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  • 159
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    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 116 (2002), S. 1839-1849 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The potential curves and spectroscopic constants of the excited states of alkali–argon diatomics MRg (M=Li, Na and K, Rg=Ar) are calculated using usual semilocal single valence electron pseudopotentials on alkali atoms [M+]-core pseudopotentials), semilocal pseudopotentials replac(ing all the electrons of argon ([Ar]-core pseudopotentials), and core polarization pseudopotentials on both centers. All states dissociating into Li(2s, 2p, 3s, 3p, 3d, 4s, and 4p), Na(3s, 3p, 3d, 4s, 4p, 4d, 5p) and K(4s, 4p, 5s, 3d, 5p, 4d, 6s, 4f, 6p, 5d, 7s, 5f) are considered. The core–core interactions for Li+Ar and Na+Ar are included using the accurate ab initio potentials of Ahmadi et al. [G. R. Ahmadi, J. Almlöf, and I. Roeggen, Chem. Phys. 199, 33 (1995); G. R. Ahmadi and I. Roeggen, J. Phys. B 27, 5603 (1994)] while the K+Ar ion data are determined by MP2 all-electron calculations. © 2002 American Institute of Physics.
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  • 160
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    The Journal of Chemical Physics 116 (2002), S. 1850-1857 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Accurate O–H(centered ellipsis)O hydrogen-bond dissociation energies were measured for the supersonic-jet-cooled complexes 1-naphthol⋅S with S=D2O, ethanol, oxirane, and oxetane. A mass-selective pump–dump–probe method was used, combining stimulated emission pumping with resonant two-photon ionization and ion-dip techniques. The ground-state dissociation energies D0(S0) are 5.83±0.13 kcal/mol for d1-1-naphthol⋅D2O, 7.94±0.02 kcal/mol for 1-naphthol⋅ethanol, 7.71±0.14 kcal/mol for 1-naphthol⋅oxirane and >8.17 kcal/mol for 1-naphthol⋅oxetane. The D0's increase by 5%–7% upon excitation of 1-naphthol to the S1 state. These dissociation energies are compared to those of the analogous complexes with S=H2O, methanol, NH3, and ND3 [Chem. Phys. Lett. 246, 291 (1996)]. The trends in D0 are compared to the electric dipole moments μ, molecular polarizabilities α¯, and gas-phase proton affinities of the H bond acceptor molecules. For the O-containing acceptors, the D0's correlate well with α¯, but the only good overall correlation for both O- and N-containing acceptors was found between the dissociation energies and proton affinities. © 2002 American Institute of Physics.
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  • 161
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    The Journal of Chemical Physics 116 (2002), S. 1858-1861 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We report an ion/neutral reaction of sympathetically laser-cooled molecular ions. Using laser-cooled fluorescence mass spectrometry, a reaction of about 100 sympathetically cooled H3O+ ions with NH3 were traced at 4 min interval as the reacting ions were kept within a radiofrequency-quadrupole ion trap. To obtain the reaction rate of the reaction molecular dynamical calculation was used to relate the fluorescence mass spectra to the quantity of the molecular ions. © 2002 American Institute of Physics.
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  • 162
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Large multiconfiguration Dirac–Fock calculations were performed for all neutral and up to 7 and 8 times ionized species of elements 107 and 108, respectively, as well as all the homologous elements Mn, Tc, Re, Fe, Ru, and Os for a number of different total angular momenta. Comparison of the resulting ionization potentials to the known values of the homologues allow an extrapolation and thus a prediction of the chemically important seven or eight ionization potentials of elements 107 or 108 as well as a prediction of some ionic radii. © 2002 American Institute of Physics.
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  • 163
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    The Journal of Chemical Physics 116 (2002), S. 1869-1876 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Photodissociation of jet-cooled, vibrationally excited CHF2Cl molecules was studied in a time-of-flight mass spectrometer to elucidate bond rupture and intramolecular dynamics. The molecules were first excited with infrared photons to the N=3, N=7/2, and N=4 C–H stretch-bend polyad components, representing stretch-bend mixed states. They were then dissociated via promotion to excited electronic states by ∼235 or 243.135 nm photons, which also tagged 35Cl(2Pj) and 37Cl(2Pj) or H photofragments, respectively, by (2+1) resonantly enhanced multiphoton ionization. Comparison of the photofragment yield spectra to the simultaneously measured room-temperature infrared absorption spectra revealed significant narrowing of the former due to the reduction of rotational inhomogeneous structure. These spectra, and particularly the band contraction, afforded observation of resonance splitting in the vicinity of the 31, 34, 41, and 44 components, reflecting redistribution times in the range of 1–18 ps. These times manifest the vibrational redistribution of the mixed states to other states of the molecule and are longer than those for the coupling of the stretch-bend. The initial vibrational excitation enhanced C–Cl and C–H bond cleavage with the former producing both ground-, Cl 2P3/2[Cl], and excited-, Cl 2P1/2[Cl*], spin-orbit states. The branching ratio of Cl*/Cl was ∼0.5 and of H/[Cl*+Cl]∼0.1, independent on the initially prepared state, signifying preferential production of Cl photofragments and energy flow from the initially excited bond. © 2002 American Institute of Physics.
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  • 164
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    The Journal of Chemical Physics 116 (2002), S. 1877-1891 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: An effective core-exciton Hamiltonian is constructed for nitroanilines which includes 1s core hole transitions of both nitrogen atoms. The wavevector and frequency dependent third order susceptibility χ(3) is calculated and used to predict the frequency-domain pump–probe spectra which show both photobleaching and excited state absorption components. Signatures of electron delocalization and differences among the para-, meta-, and ortho-isomers are discussed. © 2002 American Institute of Physics.
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  • 165
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    The Journal of Chemical Physics 116 (2002), S. 2794-2802 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Minimal Electron Nuclear Dynamics theory is applied to D2+NH3+ reaction at collision energies from 6 to 16 eV in the center-of-mass frame. This method for direct nonadiabatic dynamics describes the electrons with a family of complex determinantal wave functions in terms of nonorthogonal spin orbitals and treats the nuclei as classical particles. There are no geometrical constraints imposed on this six-atom system. Emphasis is put on the details of the abstraction and exchange reaction mechanisms for ground-state reactants. Comparisons are made to recent molecular-beam experiments. © 2002 American Institute of Physics.
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  • 166
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    The Journal of Chemical Physics 116 (2002), S. 2811-2824 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We have performed the first ab initio computational investigation of the elastic scattering of electrons by the isolated cluster which was described with its symmetry lowered (due to Jahn–Teller distortion) to the C2 and Ci point groups. The energy range considered was 1–10 eV. The geometry and electronic energy of the molecule were taken to be those of its ground state. The total and partial cross sections were calculated through a coupled-channel dynamics with inclusion of a parameter-free model exchange and correlation-polarization potentials. The scattering process has been found to exhibit a rather complex resonant structure due to the special "hollow" framework of the molecular cage. One distinguishing feature of the cross sections is the presence of strong near-threshold peaks which we attribute to a series of C20− metastable negative ions. The present results therefore provide a benchmark calculation which could be of guidance to future experiments on the very recently produced fullerene C20 species. In fact, the analysis carried out in the present work allows us to assign each scattering resonance to a specific molecular state, their symmetries and parameters obtained from our calculations. © 2002 American Institute of Physics.
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  • 167
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    The Journal of Chemical Physics 116 (2002), S. 2836-2847 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The S1↔S0 and S2↔S0 vibronic spectra of the supersonically cooled 2-pyridone dimer (2PY)2 and its 13C-, d1-, and d2-isotopomers were investigated by two-color resonant two-photon ionization and fluorescence spectroscopies. For the C2h symmetric (2PY)2–h2 and (2PY)2–d2 complexes, the 2PY moieties are equivalent and the S1↔S0 (1Ag↔1Ag) transition is forbidden. A single H/D or 12C/13C isotopic substitution reduces the symmetry to Cs, so that (2PY)2–13C and (2PY)2–d1 both exhibit S1↔S0 and S2↔S0 transitions. The S1/S2 state exciton splittings are 43.6 cm−1 and 52.4 cm−1, respectively. These are analyzed in terms of a Frenkel model and compared to calculated splittings based on ab initio monomer transition dipole moments. For (2PY)2–d1, whose 2PY subunits are different, an excitation transfer time of 318 fs is calculated from the exciton splitting. The S1↔S0 and S2↔S0 spectra are analyzed and assigned. Several bu intermolecular vibrations of S1 appear via vibronic coupling to the S2(Bu) state. Combination of the fluorescence data from excitation of the S1 and S2 origins and vibrational excitations of (2PY)2–h2, (2PY)2–d1, and (2PY)2–d2 allows the determination of the six S0 state intermolecular vibrational frequencies.© 2002 American Institute of Physics.
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  • 168
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Accurate quantum mechanical results for the thermal rate constant of the prototypical six atom reaction, CH4+H→CH3+H2, are reported in this article. Previous k(T) results for temperature values below 500 K are extended up to 1000 K. This is achieved employing a combined iterative diagonalization and statistical sampling approach for the evaluation of the flux correlation function. The accurate reaction rate data obtained for the extended temperature range is used to test several approximations related to the transition state theory. The study especially focuses on the contribution of vibrationally excited states of the activated complex to the thermal rate constant. © 2002 American Institute of Physics.
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  • 169
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    The Journal of Chemical Physics 116 (2002), S. 2852-2862 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: An accurate method to compute the annihilation rate in positronic systems by means of quantum Monte Carlo simulations is tested and compared with previously proposed methods using simple model systems. This method can be applied within all the quantum Monte Carlo techniques, just requiring the accumulation of the positron–electron distribution function. The annihilation rate of e+LiH as a function of the internuclear distance is studied using a model potential approach to eliminate the core electrons of Li, and explicitly correlated wave functions to deal with all the remaining particles. These results allow us to compute vibrationally averaged annihilation rates, and to understand the effect of the Li+ electric field on positron and electron distributions. © 2002 American Institute of Physics.
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  • 170
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    The Journal of Chemical Physics 116 (2002), S. 2879-2887 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: An ab initio adiabatic and diabatic study of the KH molecule is performed for all states below the ionic limit [i.e., K (4s, 4p, 5s, 3d, 5p, 4d, 6s, and 4f)+H(1s)] in 1Σ+ and 3Σ+ symmetries. Adiabatic results are also reported for 1Π, 3Π, 1Δ, and 3Δ symmetries. The ab initio calculations rely on pseudopotential, operatorial core valence correlation, and full valence CI approaches, combined to an efficient diabatization procedure. For the low-lying states, our vibrational level spacings and spectroscopic constants are in very good agreement with the available experimental data. Diabatic potentials and dipoles moments are analyzed, revealing the strong imprint of the ionic state in the 1Σ+ adiabatic states while improving the results. The undulations of the diabatic curves and of the triplet–singlet diabatic energy difference which we found positive, as in Hund's rule, are related to the Rydberg functions. As for LiH, the vibrational spacing of the A state is bracketed by our results with and without the improvement taking into account the diabatic representation. Experimental suggestions are also given. © 2002 American Institute of Physics.
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