Pseudorotation and the raman CH stretching spectrum of gaseous cyclopentane-d9: Local modes in a floppy molecule
The deuterium-isolated CH stretching band in the isotropic Raman spectrum of gaseous cyclopentane-d9 shows structure associated with the pseudorotation dynamics of the ring. This structure can be reproduced by a theoretical calculation that uses the adiabatic approximation to separate the CH stretching and pseudorotation motions. The uncoupled pseudorotation is modeled as a free one-dimensional rotor, and the CH stretching-pseudorotation coupling is determined from the dependence of the CH stretching frequency on the pseudorotation angle. If this CH stretching-pseudorotation coupling is estimated by assuming a linear correlation between deuterium-isolated CH stretching frequencies and ab initio CH bond lengths, a spectrum with structure much like that of the experimental spectrum is obtained; minor modification of the parameters in this coupling potential yields an excellent fit to the observed spectrum. If the coupling potential is calculated directly from the empirical potential of Lifson and Stern, the comparison with the observed spectrum is poor. Both quantum and classical treatments of the pseudorotation dynamics give similar spectra. In the classical picture, the observed spectrum can be interpreted as the superposition of many CH stretching spectra with varying degrees of motional averaging, each corresponding to cyclopentane-d9 molecules with different amounts of pseudorotational momentum. © 1992 American Chemical Society.
Variyar, JE; MacPhail, RA
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