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

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.