PSEUDOROTATION OF CYCLOPENTANE AND ITS DEUTERIATED DERIVATIVES
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The gas-phase mid-infrared Raman spectra of cyclopentane, cyclopentane-d, cyclopentane-1,1-d2, cyclopentane-1,1,2,2,3,3-d6, and cyclopentane-d10 have been recorded and analyzed in order to investigate the vibrational potential energy surface associated with the pseudorotational and radial motions. For cyclopentane two separate combination band series involving the pseudorotation and a CH2 deformation or a CH2 wag were observed. The spacings in those series were about 5.4 and 4.1 cm-1, respectively. Similar series with spacings of about 5.0 and 3.7 cm-1 were observed for the d1 species. Cyclopentane-d10 shows one combination band series near 1000 cm-1 with a spacing of 4.0 cm-1. In addition to these combination bands, pseudorotational fine structure was observed in the overtone region of the radial band (420-560 cm-1 for the different isotopic species). Previously, the absorption in this region had been mistakenly assigned to a vibrational fundamental. The radial band itself was observed for each species in its Raman spectrum. Computational methods for calculating the energy levels for this type of two-dimensional problem have been developed. Reduced mass calculations for each isotopic species have also been carried out. The observed spectra have been analyzed both by using a one-dimensional approximation model and by calculating a two-dimensional potential energy surface. The results demonstrate that the pseudorotation in cyclopentane is relatively unhindered and that the barrier to planarity is about 5.5 kcal/mol (values ranging from 1808 to 2090 cm-1 were calculated by using different models for the various isotopic species). 1988 American Chemical Society.
