The torsional spectra of the isomeric butenes. An ab initio study of the far infrared and Raman spectra of trans-2-butene, cis-2-butene and 2-methylpropene (isobutene)

Torsional energy levels of trans-2-butene, cis-2-butene and 2-methylpropene (isobutene) were calculated within the restricted Hartree-Fock approximation with Moller-Plesset perturbation theory MP2/RHF using double and triple zeta split orbitals with d and f polarization functions. The observed infrared and Raman band positions were correlated with their calculated positions, and it was concluded that, apart from some minor adjustments, the literature assignments of the fundamental and first sequence transitions are essentially correct. Whereas the terms in the potential that control the saddle point and the barrier maximum were found to be moderately sensitive to variations in the basis set, those that describe the intramolecular methyl-methyl coupling were sensitive to basis set contraction and polarization functions, The heights of the barriers (saddle point)/(barrier maxima) for trans-, cis- and iso-butenes at the level MP2/6-311 G(df,p) were found to be 700.61/1438.38, 222.04/460.80 and 736.14/1708.40 cm(-1), respectively.