Methylenecyclohexenes: inversion barriers from the far-infrared spectra

Abstract

The far-infrared spectra of 3-methylenecyclohexene, 4-methylenecyclohexene, and 1-methyl-3-methylenecyclohexene in the region of 80–400 cm−1 are reported and interpreted mainly with a view to the two lowest out-of-plane deformations of the cyclohexene ring. Typical C-type band sequences are observed and interpreted in terms of ring inversion modes. Barriers to inversion are determined by means of a one-dimensional Hamiltonian with a quadratic-quartic potential function. Possible ground state geometries and forms of the inversion coordinates are discussed and compared to those thought to occur in the analogous cyclohexenones.