THE CH STRETCHING SPECTRUM OF GASEOUS CYCLOPENTANE-D9: COUPLING OF STRETCHING AND PSEUDOROTATION
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Date
1989
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Ohio State University
Abstract
It is well known that the lowest frequency vibration in cyclopentane corresponds to pseudorotation, a rapid puckering motion along a nearly barrierless phase-angle coordinate $\phi$. This pseudorotational motion couples to the CH stretching vibrations and leads to pseudorotational structure in the CH stretching spectrum. We have obtained isotropic Raman spectra of gaseous $C_{5}D_{9}H$ which show this structure very clearly. Most of the stretch-pseudorotation coupling can be accounted for by the adiabatic variation in the CH stretching frequency with $\phi$. This frequency variation can then be used in an adiabatic approximation to simulate the CH stretching spectrum. We have calculated the frequency variation with $\phi$ by two methods, one using ab initio CH bondlengths and the other with a modified form of the Lifson-Stern empirical potential[1]. While both of these methods give simulated pseudorotational structure that is in qualitative agreement with the observed spectrum, the ab initio approach is significantly better at reproducing the spectral details. The effects of other perturbations on the spectrum have also been considered, including thermally excited radial levels and the variation of the zero point energy and reduced mass with $\phi$.
Description
$^{1}$ S. Lifson and P. Stern, J. Chem. Phys. 77, 4542 (1982).
Author Institution: Department of Chemistry, Duke University
Author Institution: Department of Chemistry, Duke University