Radical cations of cis- and trans-1,3-di- and 1,3,5-trimethylcyclohexanes. Matrix influence on two nearly degenerate SOMOs

Abstract

Radical cations of cis and trans isomers of 1,3-di- and 1,3,5-tri-methylcyclohexane, stabilized in various γ-irradiated solute–halocarbon matrices at 77 K and above, have been investigated by means of EPR spectroscopy. The hfs constants have been compared with those calculated using semiempirical methods, INDO (spin density) and MNDO (geometry optimizations). The SOMOs of the methyl-substituted cyclohexane cations are all similar to either of the orbitals, a_g or b_g, following a Jahn–Teller split of the e_g orbital (HOMO) of cyclohexane. The selection of SOMO depends on matrix and isomer. The trans isomers have one axial methyl group and the cations were found to take an a_g-like SOMO in all matrices used, with large spin density on the equatorial hydrogen attached to the axially substituted ring-carbon. Using CF₃CCl₃ as matrix the EPR signal due to the trans-1,3,5-trimethylcyclohexane cation contains (in addition) a spectrum of the b_g-like SOMO superimposed. The cation of cis-1,3-dimethylcyclohexane shows contributions from both SOMOs in the matrices CF₃-cC₆F₁₁, CF₂ClCFCl₂ and CF₃CCl₃, but exclusively a b_g-like SOMO in cC₆F₁₂. A b_g-like SOMO is observed for the cis-1,3,5-trimethylcyclohexane cation in all matrices. The abundance ratios, [a_g]/[b_g], for the cases where the matrix-stabilized cation shows contribution from both b_g- and a_g-like SOMOs simultaneously, are found to be ca. 1/4 from line shape simulations. No prominent temperature dependence of this ratio is detected.