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  • 1
    ISSN: 1434-6079
    Keywords: 31.15 ; 33.10
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract New analytical formulas for the torsionL-tensor are reported, which are used in molecular normal mode analysis calculations for the nonlinear transformation of the force constants from internal coordinates to normal coordinates. The proposed formalism exhibits a relative simplicity as compared to previous reported results. Sample calculations for methanol and hydrazine are presented.
    Type of Medium: Electronic Resource
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  • 2
    ISSN: 1434-6079
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract. The fragmentation statistics of (H2O) n and (NH3) n clusters (n = 100-1000) is investigated by MD simulations at different temperatures. The fragment size distributions are found to be well described by power laws over a wide range of excitation energies. The maximum fragment size depends linearly on the cluster size. A compact analytical model, implying the maximum fragment size and the power law exponent, is shown to fairly fit the average fragment size profiles. Measurements are carried out for the maximum fragment sizes after electron impact ionisation for water and ammonia clusters in the size range of n = 50 to 2100. The measured linear dependence on cluster size is used to estimate the fragment size distributions.
    Type of Medium: Electronic Resource
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  • 3
    ISSN: 1434-6079
    Keywords: 36.40.+d ; 33.10.Gx
    Source: Springer Online Journal Archives 1860-2000
    Topics: Physics
    Notes: Abstract A second order perturbation approach for the evaluation of the splitting and shifting of the infrared vibrational bands of molecular clusters, based on early publications of Buckingham, is presented. The Hamiltonian of the system comprises harmonic- and anharmonic intramolecular vibration terms, as well as the intermolecular potential. The anharmonic contributions of the intramolecular force field and the intermolecular potential are treated as a perturbation. In order to extend the applicability of the approach to homogeneous molecular clusters, the formalism of degenerate perturbation theory is employed. The new approach is applied to methanol clusters from dimer to hexamer for calculating the frequency shifts of the OH stretching mode (ω1=3681.5 cm−1), the CH3 rocking mode (ω7=1074.5 cm−1), and the CO stretching mode (ω8=1033.5 cm−1). The numerical results compare favourably with experimental and previous theoretical data (except for ω1), but with a tendency to overestimate the shifts.
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  • 4
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 109 (1998), S. 8295-8303 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A recently developed site–site intermolecular potential for UF6, featuring exchange, dispersion, and electrostatic terms, is used to calculate minimum energy structures of homogeneous UF6 clusters up to the decamer. The structures of mixed (UF6)2–Arn clusters are also calculated by adding appropriate interaction terms. The IR spectra corresponding to the determined cluster structures in the region of the ν3 vibrational mode of the monomer (at 627.724 cm−1) are calculated using a second-order line shift formalism, treating the anharmonic intramolecular force field and the intermolecular potential as a perturbation. The leading interaction mechanism responsible for the line shifts of the ν3 mode is found to be the electrostatic one (implicitly the resonant dipole–dipole coupling). The theoretical spectra are shown to satisfactorily describe the peaks around 623, 632, and 640 cm−1 found in the recently measured Fourier transform IR spectra in a continuous supersonic Laval nozzle flow and attributed to the clusters formed by UF6. © 1998 American Institute of Physics.
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  • 5
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 106 (1997), S. 6795-6805 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The structures of small hydrazine clusters from the dimer to the hexamer have been calculated using a standard site-site intermolecular potential and a newly developed systematic approach which is essentially based on monomer properties. Aside from the repulsive and the attractive dispersion and induction interaction special care is taken for the determination of the electrostatic interaction which is represented by a distributed multipole expansion and a penetration correction. Based on these potentials the vibrational spectra of the N-N stretching and the asymmetric NH2 wagging mode are calculated using degenerate perturbation theory. While the small shifts of the N-N stretching mode are fairly well reproduced by both potential models, large differences are predicted for the asymmetric NH2 wagging mode. Here, redshifts of –30 cm−1 are calculated for the standard and blueshifts of 100 cm−1 are obtained for the systematic potential in agreement with experiment. The analysis shows that the reason for this behavior is the careful treatment of the electrostatic term in this model. © 1997 American Institute of Physics.
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  • 6
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Vibrational predissociation spectra of hydrazine (N2H4)n clusters have been measured from the dimer to the tetramer using a linetunable, isotopically substituted CO2-laser in order to fill the frequency gap between 990 and 1010 cm−1. The clusters are size selected in a scattering experiment with helium atoms. The large blue shifts of the asymmetric NH2 wag mode at 937 cm−1 are completely interpreted by calculations based on a recently determined systematic model potential. The gross shifts of 60 cm−1 for the dimer, 80 cm−1 for the trimer, and 110 cm−1 for the larger clusters are explained by the different structures: Cyclic arrangements with two hydrogen bonds per molecule for the dimer, rings with one hydrogen bond per molecule for the trimer, and three-dimensional structures for the larger ones. The peaks in the spectra are caused by characteristic vibrations to which more than one isomer contributes. © 1997 American Institute of Physics.
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  • 7
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 106 (1997), S. 5910-5919 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A new site–site intermolecular potential model for UF6, featuring exchange, dispersion, electrostatic and induction terms, is presented. The new potential, with the parameters adjusted according to the observed monomer transition dipole moment and reproducing the experimental temperature dependence of the second virial coefficient, is used to determine UF6 cluster structures up to the hexamer and, by means of a second order line shift formalism, to calculate the corresponding IR-spectra in the region of the ν3 vibrational mode (at 627.724 cm−1). The contributions of the various potential terms to the frequency shifts are analyzed and the leading interaction mechanism is found to be the resonant dipole–dipole coupling. The theoretical spectra are compared and interpreted against recent Fourier transform IR-spectroscopy measurements. © 1997 American Institute of Physics.
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  • 8
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 103 (1995), S. 6394-6413 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A new second order perturbation approach for evaluating the splittings and shifts of the vibrational bands of homogeneous molecular clusters, consistently treating degenerate normal modes, is described. The Hamiltonian of the system comprises harmonic and anharmonic intramolecular vibration terms, and the intermolecular potential. The anharmonic intramolecular contributions and the intermolecular potential are treated as a perturbation. A new site-site intermolecular potential model for SF6, featuring exchange, dispersion, electrostatic and induction terms, is presented. The new potential, with the parameters adjusted according to the observed monomer transition dipole moment and reproducing the experimental temperature dependence of the second virial coefficient, is used to determine SF6 cluster structures up to the hexamer and, by means of the new line shift formalism, to calculate the corresponding IR-spectra in the region of the ν3 vibrational mode (at 947.968 cm−1). The contributions of the various potential terms to the frequency shifts are analyzed and the leading interaction mechanism is confirmed to be the electrostatic one (implicitly the resonant dipole-dipole coupling). The theoretical spectra are shown to fairly describe the experimental evidence when considering only exchange, dispersion and electrostatic interactions. With the available atomic polarizabilities, induction seems to lead to a systematic redshift of the entire spectrum for all cluster sizes. The structure of the cluster vibrations is investigated in terms of the individual monomer vibrations and is correlated with the found geometrical cluster configurations. © 1995 American Institute of Physics.
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  • 9
    Electronic Resource
    Electronic Resource
    Amsterdam : Elsevier
    Physics Letters A 93 (1983), S. 223-226 
    ISSN: 0375-9601
    Source: Elsevier Journal Backfiles on ScienceDirect 1907 - 2002
    Topics: Physics
    Type of Medium: Electronic Resource
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