Electronic structure, methyl group reorientation and reactions of radical cations of 1,2,4-trimethylcyclohexanes: an EPR study

Abstract

Two isomeric radical cations of 1,2,4-Me₃-cyclohexane, r-1,t-2, t-4-Me₃-c-C6⁺ and r-1, t-2, c-4-Me₃-c-C6⁺, have been studied at cryogenic temperatures adopting the halocarbon matrix technique. Both isomer cations show the same EPR hyperfine (hf) pattern with a triplet of triplets with a₁^H= 60 G (2 H) and a₂^H= 32 G (2 H) at 4.2 K. The hf parameters coincide with those of the cis and trans isomers of the 1,2-Me₂-c-C6 cation, implying a similar electronic ground state structure in which the main part of the unpaired electron resides in the C-1–C-2–σ-bond. The EPR spectra show a reversible temperature change between ca. 77 and 180 K. The temperature-dependent spectra can be explained by a dynamic effect in which the methyl groups on C-1 and C-2 rotate. Adopting a three-site jump model the rate constants were evaluated to be ca. 5.0 × 10⁶ s^–1(100 K) to ca. 5.0 × 10⁷ s^–1(180 K) with an activation energy of 1.1 kcal mol.^–1 This low activation energy is compared to methyl rotation in radical cations of propane⁺ and butane⁺, and is discussed in terms of the unpaired electron distribution within the framework of carbon atoms. Furthermore, it is shown that secondary and tertiary alkyl radicals are predominantly formed by thermal reactions of r-1, t-2, t-4-Me₃-c-C6⁺ and r-1, t-2, c-4-Me₃-c-C6⁺, respectively, in the CF₃–c-C₆F₁₁ matrix.