Analysis of torsional spectra of molecules with two internal C3V rotors. IX. The torsional potential functions of isobutene-d and -d6

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Abstract
The Raman spectra of gaseous isobutene (CH3)2C=CH2 and isobutene‐d6 at a resolution of 1 cm−1 have been recorded between 100 and 500 cm−1. The far infrared spectra of the same compounds have been recorded over the same frequency range with a resolution of 0.5 cm−1. Several torsional transitions have been observed for both molecules and these data have been used to determine the torsional potential function based on a semirigid model. For the light molecule the average effective barrier was found to be 728±3 cm−1 (2.08±0.01 kcal/mole), and the cosine–cosine coupling term V33 has a value of 165±14 cm−1 (0.51±0.04 kcal/mole), and the sine–sine term V′33, has a value of −135±4 cm−1 (−0.38±0.01 kcal/mole). For the heavy molecule the effective barrier was found to be 719±6 cm−1 (2.05±0.02 kcal/mole) with a V33 of 49±17 cm−1 (0.14±0.05 kcal/mole) and a V′3 of −135±2 cm−1 (−0.38±0.01 kcal/mole). The barrier value compares very well with the average value of 2.21 kcal/mole reported from the microwave splittings. Comparisons are also given to similar quantities obtained for other molecules.

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