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Intramolecular Energy Transfer

Intramolecular Vibrational Energy Redistribution and Torsional Isomerization: A Model Classical and Quantum Study


A simple model of the vibrational dynamics of ABBA type sequentially bonded tetra-atomic molecules is investigated by classical and quantum mechanical methods. The model Hamiltonian excludes bond stretching and asymmetric bending but includes the kinematic coupling between the torsional motion and symmetric bond bending which results in nonlinear resonances. The effect of this coupling on energy levels and the timescale of intramolecular vibrational energy redistribution (IVR) is evaluated and discussed in terms of both resonant and nonresonant effects. The rate of torsional isomerization is compared to the prediction of Transition State Theory, and related to the observed IVR.

For more information see the electronic paper and the computer animations.

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IVR amongst the Ring Modes of S0 Benzene

How energy moves around the vibrational modes of a polyatomic molecule, such as benzene, is of considerable interest, both spectroscopically and theoretically. Much effort has been devoted to observing and understanding how energy is transferred out of initially excited overtone states involving the CH stretching modes. Significantly less study has been devoted to an understanding of how and on what timescale energy is transferred amongst the ring modes. Such low frequency modes dominate the vibrational state density and also play a role in the irreversible nature of the decay out of C-H modes. The motivation for the present classical trajectory study, is to model the results of recent experimental observations regarding the extent and timescale of IVR involving the ring modes. The linewidths found experimentally were instrument limited at 1 cm-1 for a range of excited ring modes for excitations of between 1200 and 8200 cm-1 yielding an upper limit on the IVR rate of 0.094 ps-1. This result is consistent with the results of our trajectory calculations which reveal an initially rapid decay followed by slow IVR at longer times.
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Last revised Wednesday 18 February 1998 EST - Harold W. Schranz Email: Harold.Schranz@anu.edu.au