Electron Spin Resonance of the Mono-tert-butyltropenyl and Tri-tert-butyltropenyl Radicals: A Study of Vibronic Near Degeneracy

Abstract

The mono-tert-butyltropenyl and tri-tert-butyltropenyl radicals have been prepared by thermal and photochemical cleavage of di-tert-butylbitropenyl and hexa-tert-butylbitropenyl. The syntheses of these dimers are reported. Electron spin resonance spectra of the tert-butyltropenyls (tert-butyl-2,4,6-cycloheptatrien-1-yls) have been investigated. Proton hyperfine splittings of both radicals are markedly temperature dependent. In addition, for tri-tert-butyltropenyl, the g value and the linewidth are temperature dependent and the proton splittings vary slightly with solvent. The experimental temperature variations are ascribed to vibronic near degeneracy in the tert-butyltropenyls, and a model is proposed in order to calculate these effects. Approximate spin densities for the two nearly degenerate states are given by ρkkπ ≈ (1 / 7)[1 ∓ P cos(8πk / 7)], where the minus sign corresponds to the ground state, ρkkπ is the diagonal element of the pi-spin-density matrix for carbon atom k, and P is a parameter which combines the effects of electron correlation and vibronic coupling. Splitting calculations using these values of ρkkπ and McConnell's relationship lead to good agreement with experiment. For tri-tert-butyltropenyl it is found that P = 1.0 ± 0.1, and the energy splitting between nearly degenerate states (ΔE) is approximately 200 ≤ ΔE ≤ 300 cm− 1. In the case of mono-tert-butyltropenyl a range of 100 ≤ ΔE ≤ 180 cm− 1 corresponds to P = 1. Consideration of the Colpa–Bolton and Giacometti–Nordio–Pavan extensions of McConnell's relationship leads to negligible modifications in the hyperfine splittings. On the basis of Hückel MO calculations in which the alkyl-group perturbation is treated as an inductive effect, it is concluded that the tert-butyl group does not appreciably affect the pi-electron wavefunction but does modify the Coulomb integral on the adjacent carbon by approximately − 0.02 β. It is found that the temperature dependence of the unresolved tert-butyl proton splittings gives rise to a major contribution to the temperature variation of the linewidth in tri-tert-butyltropenyl. Analysis of the temperature dependence of the g value of this radical on the basis of a simple two-level scheme leads to approximate values for (gE–gG) and gG, where gG and gE designate the g values in the ground and excited vibronic levels.