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  • 1
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 95 (1991), S. 106-120 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Intramolecular energy transfer rates and pathways in disilane Si2H6 have been investigated in detail by analysis of the envelope functions of the time variation of the uncoupled normal-mode kinetic energies [J. Chem. Phys. 89, 5680 (1988)] and by a new method that involves the Fourier transform of the local-mode "bond energies.'' The results show that the total intramolecular vibrational relaxation (IVR) rate out of a given mode is generally much faster than the total dissociation rate. However, many of the individual mode-to-mode rate coefficients are significantly smaller than this rate. Consequently, IVR is not globally rapid on the time scale of the reactions. The Si–Si and local modes relax over a much longer time scale than the Si–H modes. This observed decoupling of sets of internal modes is interpreted to mean that phase space is not explored ergodically on the time scale of the reactions, even at internal energies significantly greater than the dissociation thresholds. The present results are consistent with and complementary to our earlier observation of trajectory rate coefficients that are considerably larger than corresponding statistical phase space predictions computed on the same potential-energy surface [J. Chem. Phys. 94, 0000 (1991)]. As a consequence, we find numerous mode-specific effects present in the system. Trajectory rates are found to be very sensitive to the nature of the initial energy partitioning. The computed kinetic isotope effects also show evidence of mode-specific chemistry. These data are consistent with the principle that a total intramolecular energy transfer rate from a given vibrational mode that is fast relative to the unimolecular reaction rate is not a sufficient condition to ensure statistical behavior and an absence of mode-specific chemistry.
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  • 2
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 95 (1991), S. 8056-8064 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A classical trajectory surface-hopping study of the effects of initial mode excitation on the spin-forbidden predissociation in a model of N2O reveals mode specificity. Incomplete energy randomization together with significant variation in energies of the surface crossing points with molecular geometry lead to changes in the accessibility of the dissociation channel and hence the rate of reaction when different modes are excited. The variations in the predissociation rate are most marked at relatively low energies. At higher energies, although energy randomization is still incomplete, there is less dispersion in the rate constants. This is attributed to the fact that at high energies a larger percentage of the crossing points become accessible and thus the directionality of the initial motion is less relevant to the rate.
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  • 3
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The unimolecular dissociation reactions of the 2-chloroethyl radical involving C–H and C–Cl bond fissions are investigated using classical trajectories and two variational transition-state theory methods on the same potential-energy surface. The transition-state theory methods employed are the efficient microcanonical sampling-transition state theory method, previously used to study the decomposition dynamics of disilane and 1,2-difluoroethane, and a J-conserving variant of this method that introduces constraining equations in the efficient microcanonical sampling procedure, such that the sampling is restricted to phase-space points associated with both a constant value of the system energy and total angular momentum.The results demonstrate that the unimolecular dissociation of the 2-chloroethyl radical is well described by statistical theories that assume an equal weight for all energetically accessible phase-space points. The results obtained from the statistical calculations form upper bounds to the trajectory-computed rate coefficients as expected for a statistical system. In addition, there is no evidence of mode-specific dynamics present in the trajectory results. The statistical behavior of the 2-chloroethyl radical stands in sharp contrast to the dissociation dynamics of disilane and 1,2-difluoroethane which have previously been shown to exhibit pronounced nonstatistical effects. It is shown that the existence of nonstatistical behavior cannot, in general, be qualitatively predicted from energy considerations alone.Comparison of the 2-chloroethyl radical, 1,2-difluoroethane, and disilane results again demonstrates that the existence of an energy decay rate out of a given bond that is fast relative to the unimolecular reaction rate is not a sufficient condition to guarantee statistical dynamics. It is found that the statistical behavior observed for 2-chloroethyl is due, in large part, to an increase in the potential coupling between the dissociating atom and the beta-carbon that occurs as the bond breaks. This coupling is associated with the conversion of the C–C single bond to a C(large-closed-square)C double bond upon C–Cl or C–H bond fission in 2-chloroethyl. It is concluded that unimolecular reactions will tend to exhibit nonstatistical dynamics if (1) the internal energy is close to the dissociation threshold, (2) motion along the reaction coordinate does not produce large energetic changes in one of more bonds in the remainder of the molecule, and (3) there exists a formation coordinate for the activated reactant that is strongly coupled to the dissociation coordinate but only weakly coupled to the other internal coordinates of the molecule.
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  • 4
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The dynamics of silicon-atom diffusion on the dimer–adatom-stacking fault model (DAS) of the reconstructed Si(111)-(7×7) surface suggested by Takayanagi et al. have been investigated using variational phase-space theory methods. The site-to-site jump frequency is obtained from the variationally minimized total flux across a right cylindrical dividing surface whose cross section in the surface plane is formed from straight line and elliptical segments. This minimized flux is corrected for surface recrossings by the computation of trajectories starting from phase-space points in the transition-state region that are obtained in the Markov walk used to evaluate the phase-space integrals in the expression for the total classical flux. The jump frequencies are used as input to the set of differential equations that describes the diffusion rates on the DAS surface. Values of the diffusion coefficient D are computed from the slopes of plots of the time variation of the root-mean-square displacements obtained from the solution of the rate equations. Arrhenius plots of the results at 300, 600, and 1000 K yield D=0.124 exp[−2.18 eV/kT] cm2/s. These rates are orders of magnitude smaller than the corresponding rates we have previously obtained for silicon-adatom diffusion on the Binnig et al. model of the Si(111)-(7×7) surface. In addition, it is found that the diffusion pattern on the DAS surface is uniform with no preferential directions for silicon-atom flow. In contrast, diffusion on the Binnig surface was found to occur via gateways at three of the four corners of the unit cell. This led to preferential directions for adatom flow. These differences lead us to suggest that careful measurements of silicon-adatom diffusion rates on the Si(111)-(7×7) surface may be a very sensitive measure of the extent to which these surface models accurately describe the experimental Si(111)-(7×7) reconstruction.
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  • 5
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 94 (1991), S. 4219-4229 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: An efficient implementation of microcanonical, classical variational transition-state theory based on the use of the efficient microcanonical sampling (EMS) procedure is applied to simple bond fissions in SiH2 and Si2H6 using recently constructed global potential-energy surfaces. Comparison is made with results of trajectory calculations performed on the same potential-energy surfaces. The predictions of the statistical theory agree well with and provide an upper bound to the trajectory derived rate constants for SiH2→SiH+H. In the case of Si2H6, agreement between the statistical theory and trajectory results for Si–Si and Si–H bond fission is poor with differences as large as a factor of 72. Moreover, at the lower energies studied, the statistical calculations predict considerably slower rates of bond fission than those calculated from trajectories. These results indicate that the statistical assumptions inherent in the transition-state theory method are not valid for disilane in spite of the fact that many of the mode-to-mode rate constants for intramolecular energy transfer in this molecule are large relative to the Si–Si and Si–H bond fission rates. There are indications that such behavior may be widespread among large, polyatomic molecules.
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  • 6
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 6607-6619 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The formation and subsequent decay of Si4 complexes as well as the direct exchange and abstraction processes in Si+Si3 collisions have been studied using quasiclassical trajectories on a new global Si4 potential energy surface fitted to available experimental and ab initio data, and on Bolding and Andersen's (BA) recently formulated silicon potential for arbitrary cluster sizes. Cross sections for Si4 formation, σf(Et), were computed as a function of initial relative translational energy Et over the range 0.01 to 4.0 eV, with the Si3 internal energy described by the Boltzmann distribution at 800 K. The cross section was found to peak sharply near Et=0, as expected, and to fall off linearly at high energy. An analytical expression for kf(T), the thermal rate constant for Si4 formation, was found by averaging σf(Et) over the Maxwell–Boltzmann distribution for Et.The analytical values of kf(T) lie between 6×1014 and 8×1014 cm3/mol s for the range 800–1500 K, and are in excellent accord with trajectory calculations of kf at 800 and 1200 K. Unimolecular dissociation rate constants for Si4, kd, were calculated as a function of Et over the 0.4 to 4.0 eV. The values of kd are well described by the RRK expression, with a value of 4.67 for the effective number of vibrational modes. Averaging the dissociation rate constant over the Maxwell–Boltzmann distribution yields an average Si4 lifetime of 413 ps at 800 K, which is not long enough for a stabilizing collision to occur at pressures characteristic of low-pressure CVD experiments. The direct exchange reaction is found to be unimportant for Et less than 1 eV, since for lower relative energies essentially all reactions proceed indirectly via Si4 complex formation. Direct atomic abstraction is energetically forbidden, on average, for Et less than 0.9 eV, and is unlikely for Et less than 2 eV. At higher energies, the end-atom exchange and abstraction channels, which are statistically favored over the apex-atom channels, are dynamically favored as well.When exchange or abstraction proceeds indirectly via an Si4 intermediate, the distinction between apex-atom, end-atom, and no-reaction channels is lost. Both the direct and indirect pathways leave a large fraction of the energy and angular momentum in the reaction products. Cross sections for Si4 formation on the BA surface are smaller than those on the global Si4 surface due to the cutoff function in the BA two-body potential terms; Si4 dissociation rates for total energies between 1.3 and 2 eV above threshold agree to within a factor of 2.3 or better with corresponding values for the Si4 surface.
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  • 7
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 92 (1990), S. 1069-1082 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The unimolecular dissociation dynamics of disilane are investigated using classical trajectory methods with a global potential-energy surface fitted to the available experimental data and the results of various ab initio calculations. The potential surface is written as the sum of 52 many-body terms containing 86 adjustable parameters which are fitted to experimental and/or calculated data for stationary point geometries, fundamental vibrational frequencies, reaction endo- and exothermicities, and potential-energy barrier heights for reactions of disilane and molecules derived from disilane. In general, the equilibrium bond lengths and angles for Si2 H6 , Si2 H5 , H3 Si–SiH, H2 Si=SiH2, H2 Si=SiH, H2 Si=Si, HSi=Si, Si2 , H2 , and SiH2 given by the global potential agree with ab initio results to within 0.03 A and 2°, respectively, or better. The predicted heats of reaction for 13 reactions involving disilane or its derivatives are in good accord with the experimental and ab initio results.The average absolute deviation is 3.55 kcal/mol. The average absolute difference between the normal-mode frequencies given by the global potential for Si2 H6 , Si2 H5, and H3 Si–SiH and those obtained from scaled MP4 calculations are 58.7, 52.1, and 62.8 cm−1 , respectively. If two low-frequency Si–Si–H deformation modes for each of these molecules are omitted from consideration, the average absolute differences are all in the range 34–36 cm−1 . The calculated barrier height for the hydrogen-atom transfer process leading to SiH4 +SiH2 products is 56.7 kcal/mol. For three- and four-center H2 elimination reactions, the barrier heights given by the global surface are 60.1 and 91.1 kcal/mol, respectively. These values are all within 1.2 kcal/mol of the results obtained by Ho et al. from MP4 calculations. The Si2 H6 dissociation dynamics at seven internal energies ranging from 5.31 to 9.31 eV have been investigated. At low internal energy, dissociation leading to SiH4 +SiH2 dominates the dynamics.At internal energies in the range 5.31≤E≤6.31 eV, the various Si2 H6 decomposition channels are, in order of importance, hydrogen-atom transfer leading to SiH4 +SiH2 , Si–Si bond rupture giving two SiH3 radicals, three-center H2 elimination, Si–H bond rupture to give Si2 H5 +H, and four-center H2 elimination to give H2 Si=SiH2 . At higher internal energies, entropy effects cause an inversion of this ordering such that Si–Si bond rupture becomes the major decomposition channel followed by three-center H2 elimination, SiH4 +SiH2 formation, Si–H bond rupture, and four-center H2 elimination. The present results suggest that the formation of disilene in disilane pyrrolysis occurs predominantly via the formation of H3 Si–SiH from three-center H2 elimination followed by a low-barrier hydrogen transfer process. For all decomposition channels, most of the available energy is partitioned in vibrational modes of the products. To a large extent, product energy partitioning is found to be governed by statistical considerations. Exceptions to this generalization are found in three-center H2 elimination and for any product which involves the formation of a new bond. We find that while three-center H2 elimination is a concerted reaction, it probably does not occur along a symmetric pathway. Hydrogen transfer to form SiH4 +SiH2 is found to be a concerted process, but four-center H2 elimination involves the rupture of one Si–H bond followed by hydrogen transfer and a subsequent H2 abstraction reaction to give the H2 +H2 Si=SiH2 products.
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  • 8
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The perturbation−trajectory method (PTM) of Jezercak, Agrawal, Thompso and Raff for application to gas−surface collision processes is commented upon. The authors contend that the PTM does not function as a true thermal reservoir or heat bath. (AIP)
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  • 9
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 92 (1990), S. 313-319 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Classical trajectories have been used to investigate intramolecular energy transfer in toluene for a potential-energy surface based on the ab initio force field by Xie and Boggs [J. Comp. Chem. 7, 158 (1986)]. Various CH bond stretching local modes were initially excited to the v=6 state. The calculated relaxation time of an excited methyl–CH stretching mode is approximately twice that of an excited ring–CH stretching mode, in qualitative agreement with experiment. The time scale of the energy transfer from the methyl group to the ring or from the ring to the methyl group is approximately 3 to 5 times longer than that of the relaxation of the excited CH bonds. The pathways of the intramolecular energy transfer were examined.
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  • 10
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 2299-2307 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Intramolecular vibrational energy redistribution and intramolecular cis→trans and trans→cis conversions have been investigated as a function of initial excitations of C–H local stretch modes and some bending normal modes in methyl nitrite by using quasiclassical trajectories. Harmonic and anharmonic valence force-field potential-energy surfaces with parameter values based on the available experimental and ab initio results were used in the calculations. The anharmonic potential gives rates of energy transfer and isomerization that are significantly faster than those for the harmonic potential. The rate of cis→trans isomerization is significantly larger than the rate of trans→cis. The energy flow out of excited C–H local stretch modes is irreversible and essentially complete in less than 0.5 ps.
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