Translational and rotational excitation of the CO 2 (00 0 0) vibrationless state in the collisional quenching of highly vibrationally excited perfluorobenzene: Evidence for impulsive collisions accompanied by large energy transfers J. Chem. Phys. 106, 7055 (1997) Classical trajectory calculations of the rate of collisional energy transfer between a bath gas and a highly excited polyatomic method, and the average energy transferred per collision, as functions of the bath gas translational energy and temperature, are reported. The method used is that of Lim and Gilbert [J. Phys. Chem. 94, 72 (1990)], which requires only about 500 trajectories for convergence, and generates extensive data on the collisional energy transfer between Xe and azulene, as a function of temperature, initial relative translational energy (E T)' and azulene initial internal energy (E'). The observed behavior can be explained qualitatively in terms of the Xe interacting in a chattering collision with a few substrate atoms, with the collision duration being much too brief to permit ergodicity but with a general tendency to transfer energy from hotter to colder modes (both internal and translational). At thermal energies, trajectory and experimental data show that the root-mean-squared energy transfer per collision, (ali 2) 112, is relatively less dependent on E' than the mean energy transfer (ali). The calculated temperature dependence is weak: (AE 2) 112 0:: To. ,corresponding to (AE down ) 0:: To.23 • Values for the calculated average rotational energy transferred per collision (data currently only available from trajectories, and required for falloff calculations for radical-radical and ion-molecule reactions) are of the order of k B T, and similar to those for the internal energy; there is extensive collision-induced internal-rotational energy transfer. The biased random walk "model B," as discussed in text, is found to be in accord with much of the trajectory data and with experiment. This suggests that energy transfer is through pseudorandom mUltiple interactions between the bath gas and a few reactant atoms; the "kick" given by the force at the turning point of each atom-atom encounter governs the amount of energy transferred. Moreover, a highly simplified version of this model explains why average energies transferred per collision for simple bath gases have the order-of-magnitude values seen experimentally, an explanation which has not been provided hitherto.
Average energy-transfer values are reported from trajectory studies of Xe and He colliding with highly excited azulene-do and azulene-d8. The calculated isotope effect is small, in agreement with experiments on related systems. This suggests that the low-frequency modes are important in determining the amount of energy transfer. Furthermore, the fact that both calculated and observed changes with deuteration are small for helium suggests that the classical description of the energy transfer is adequate. The biased random walk model for collisional energy transferred is found to reproduce the observation of a small isotope effect. The calculated fraction and magnitude of supercollisions are in accord with extant data and are predicted to be relatively unaffected by deuteration. IntroductionTrajectory calculations can be used to make qualitative and quantitative deductions on the nature of the distribution functions for the rate of collisional energy transfer that are necessary for the interpretation and prediction of unimolecular and recombination rate data.'-3 Comparison with experimental data for isotopic substitution is a useful tool for the theoretical dynamics, as it changes dynamical properties of the system without affecting the potential energy surface. Hence, extra information can be obtained with little extra effort. (It is even possible to use nonphysical isotopes to test mechanistic ideas.) The trajectory study of deuteration effects given in this paper supplement our earlier trajectory studies of collisional energy transfer of highly excited substrate interacting with a bath g a~.~-~ In those studies we used established methods to examine the details of collisions of monatomic bath gases with highly excited azulene.Theoretically, the field is still far from fully understood; trajectory methods, while showing promise, have been able to reproduce some, but by no means all, the results of "direct" experimenh2 Although it is possible to reproduce some trends and values measured experimentally, a worrying discrepancy occurs with the behavior of lighter bath gases He or Ne.' Without agreement over the range of bath gases an important advantage of trajectory techniques is partially lost-the ability to obtain answers for much more specific questions about the intimate processes that occur in collisions.A number of possible explanations have been suggested for the discrepancies, as follows.(1) There is the prospect of inaccuracies in the potential energy surface for the light bath gases. It seems likely that the Lennard-Jones repulsions currently used are too 'hard" (e.g., refs 8 and 9), and it has been shown both from trajectories7 and from model theoryIO that softening the repulsive part of the potential significantly decreases the calculated energy-transfer values.(2) Gilbert and Zare" suggested that dynamical quantum effects could be behind the failure of the classical calculations. The effects arise from the same interference of partial waves that make the quantum scattering cross sections finite, and the...
Classical simulations are used to examine vibrational energy redistribution in a chain of hydrogen bonded N-methylacetamide molecules following excitation of a NH stretching mode or Amide-I mode at one end of the chain. The dynamics take place on a complex potential energy surface constructed from ab initio calculations and empirical surfaces, refined with reference to observed vibrational frequencies. Simulations are used to characterize the dynamical effects of variation of several important potential parameters, and to determine the existence of coherent energy transfer by Fermi resonance mechanisms. Results are discussed with reference to simple models.
A simple model is presented to describe a mechanism by which large quanta of vibrational energy may be transferred coherently through a hydrogen bonding network. Exact results and numerical simulations of both the quantum and classical dynamics are presented. The physical context of the model is briefly described and order of magnitude estimates of the energy transfer time scale are discussed.
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