ELECTRON SPIN RESONANCE OF MONO-TERT-BUTYLTROPENYL AND TRI-TERT-BUTYLTROPENYL RADICALS - A STUDY OF VIBRONIC NEAR DEGENERACY

The mono-fert-butyltropenyl and tri-tert-butyltropenyl radicals have been prepared by thermal and photochemical cleavage of di-fert-butylbitropenyl and hexa-tert-butylbitropenyl. The syntheses of these dimers are reported. Electron spin resonance spectra of the fert-butyltropenyls (tert-butyl-2,4,6- cycloheptatrien-l-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 teri-butyltropenyls, and a model is proposed in order to calculate these effects. Approximate spin densities for the two nearly degenerate states are given by pkkπ≈ (1/7)[1∓P cos(8πk/7)], where the minus sign corresponds to the ground state, pkk π 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 p kkπ 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 mano-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 pielectron 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-fert-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-g G) and gG, where gG and gE designate the g values in the ground and excited vibronic levels.