Analysis of torsional spectra of molecules with two C3v internal rotors. XIV. c i s- and t r a n s-2-butene

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Abstract
The low frequency Raman and far infrared spectra of gaseous cis‐ and trans‐2‐butene have been recorded from 70–450 cm−1. Considerable torsional data have been obtained for the trans‐2‐butene molecule with the Au torsional fundamental being observed at 163.6 cm−1 in the infrared spectrum. The overtone of the Bg fundamental was observed in the Raman spectrum at 432 cm−1. The torsional data have been used to determine the potential function based on a semirigid model and the effective barrier was found to be 713 cm−1 (2.04 kcal/mole). The cosine–cosine coupling term, V33, was found to be zero whereas the sine–sine coupling term, V33', has a value of −48 cm−1 (−0.14 kcal/mole) and V60=V06=−4 cm−1 (−0.01 kcal/mole). For the cis‐2‐butene molecule only the B2 torsional mode was observed at 101.0 cm−1 which gives an effective barrier of 250 cm−1 (0.87 kcal/mole). These data are compared to the corresponding quantities obtained earlier by other methods.

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