PSEUDOROTATIONAL DYNAMICS OF LIQUID CYCLOPENTANE FROM CH STRETCHING SPECTRA

Author Institution: Department of Chemistry, Duke UniversityRecent experimental and theoretical studies in our laboratory have shown that deuterium isolated CH stretching vibrations can be used to probe rapid conformational dynamics of molecules in condensed phases. These isolated CH stretching frequencies are sensitive to the local conformation of the CH bond, and thus to the conformational dynamics which interchange bond types on a fast enough timescale. Our work has focused on the ultrafast internal dynamics of cyclopentane. This molecule undergoes a nearly barrierless conformational rearrangement called pseudorotation that interchanges the CH oscillators on the subpicosecond timescale. We have observed this motion as It is reflected in the partial motional collapse of the CH stretching bands in C5D9H. Analysis of these spectra should help us understand the effect of a solvent on low-barrier conformational dynamics in liquids. To describe pseudorotation and its effects on the observed CH stretching spectrum, stochastic differential equations describing coupled vibration and pseudorotational motions are solved numerically and fit to isotropic Raman and FTIR spectra. The results of various stochastic models, in particular extended diffusion will be discussed. From fits to the experimental spectra we have obtained the temperature dependence of dynamical parameters such as the pseudorotation angular momentum relaxation time. We find the dynamical parameters to be independent of viscosity over the entire temperature range studied implying the dynamics along this coordinate cannot be described by a Stokes-Einstein model