Radical cations of cyclohexanes alkyl-substituted on one carbon: an ESR study of the Jahn–Teller distorted HOMO of cyclohexane

Abstract

Cation radicals of cyclohexanes alkyl-substituted on one carbon have been stabilized in perfluoromethylcyclohexane and other halocarbon matrices at 4.2 K and studied by means of ESR spectroscopy. It was found that all have an electronic ground state resembling the ²A_g state of the cyclohexane cation, one of the possible states following a Jahn–Teller distortion of the D_3d cyclohexane chair structure. The cations can be classified into two groups depending on the substituted alkyl group. To the first group belong the cations with a methyl group or a primary carbon (ethyl, n-propyl or isobutyl group) attached to the ring. The disubstituted cyclohexane cations of 1,1-dimethylcyclohexane and 1-methyl-1-ethylcyclohexane were also found to have a similar structure. The ESR spectra are characterized by a 1:2:1 three-line pattern with the hyperfine. (hf) splitting due to two magnetically equivalent equatorial ring hydrogens. The magnitude of the splitting was found to depend on the size and number of substituents, ranging from 74 G (methylcyclohexane˙⁺) to 55 G (isobutylcyclohexane˙⁺). An additional doublet, 17–34 G, due to a hydrogen on the substituent could be detected in certain cases. Such hydrogens are axial with one of the elongated C–C bonds in the ring structure which contains a relatively large fraction of the unpaired electron. It follows that the substituents are located asymmetrically with respect to an a_g-like SOMO in the ring. In the second group a secondary or tertiary carbon connects the substituent to the ring, such as an isopropyl or tert-butyl group. The largest hf splittings are ca. 30 G in magnitude, due to certain hydrogens on the substituent which are axial with respect to the cyclohexyl bond. It follows that an a_g-like SOMO in the ring here is symmetrically arranged with respect to the position of the substituent. Hyperconjugation is the dominating mechanism for the spin transfer in all cations reported in this study.