| Abstract
| - With the aid of a reported inversion splitting value, the far-infrared spectrum resulting from the ring-puckeringvibration of coumaran has been reassigned and the one-dimensional potential energy function has beendetermined. The barrier to planarity is 155 ± 4 cm-1 and the dihedral angle is 25°. These results agree wellwith the millimeter wave spectra values of 152 cm-1 and 23°, which utilized different data and a differenttype of potential function for the calculations. The MP2/cc-pvtz ab initio values of 238 cm-1 and 26.5° agreemore poorly. If the benzene ring is assumed to remain rigid, the calculated barrier drops to 204 cm-1. Thepuckering potential functions for the ring-flapping and ring-twisting vibrationally excited states were alsodetermined and the barriers were found to be 149 and 156 cm-1, respectively.
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