Electron Spin Resonance of the Monohyphen;terthyphen;butyltropenyl and Trihyphen;terthyphen;butyltropenyl Radicals: A Study of Vibronic Near Degeneracy

摘要

The monohyphen;terthyphen;butyltropenyl and trihyphen;terthyphen;butyltropenyl radicals have been prepared by thermal and photochemical cleavage of dihyphen;terthyphen;butylbitropenyl and hexahyphen;terthyphen;butylbitropenyl. The syntheses of these dimers are reported. Electron spin resonance spectra of theterthyphen;butyltropenyls (terthyphen;butylhyphen;2,4,6hyphen;cycloheptatrienhyphen;1hyphen;yls) have been investigated. Proton hyperfine splittings of both radicals are markedly temperature dependent. In addition, for trihyphen;terthyphen;butyltropenyl, thegvalue and the linewidth are temperature dependent and the proton splittings vary slightly with solvent. The experimental temperature variations are ascribedto vibronic near degeneracyin theterthyphen;butyltropenyls, and a model is proposed in order to calculate these effects. Approximate spin densities for the two nearly degenerate states are given byrgr;kkpgr;thinsp;ap;thinsp;lpar;1thinsp;sol;thinsp;7rpar;lsqb;1thinsp;mnplus;thinsp;Pcoslpar;8pgr;kthinsp;sol;thinsp;7rpar;rsqb;, where the minus sign corresponds to the ground state,rgr;kkpgr;is the diagonal element of the pihyphen;spinhyphen;density matrix for carbon atomk, andPis a parameter which combines the effects of electron correlation and vibronic coupling. Splitting calculations using these values ofrgr;kkpgr;and McConnell's relationship lead to good agreement with experiment. For trihyphen;terthyphen;butyltropenyl it is found thatPthinsp;equals;thinsp;1.0thinsp;plusmn;thinsp;0.1, and the energy splitting between nearly degenerate stateslpar;Dgr;Erpar;is approximately200thinsp;le;thinsp;Dgr;Ethinsp;le;thinsp;300cmminus;thinsp;1. In the case of monohyphen;terthyphen;butyltropenyl a range of100thinsp;le;thinsp;Dgr;Ethinsp;le;thinsp;180cmminus;thinsp;1corresponds toPthinsp;equals;thinsp;1. Consideration of the Colpandash;Bolton and Giacomettindash;Nordiondash;Pavan extensions of McConnell's relationship leads to negligible modifications in the hyperfine splittings. On the basis of Huuml;ckel MO calculations in which the alkylhyphen;group perturbation is treated as an inductive effect, it is concluded that theterthyphen;butyl group does not appreciably affect the pihyphen;electron wavefunction but does modify the Coulomb integral on the adjacent carbon by approximatelyminus;thinsp;0.02 bgr;. It is found that the temperature dependence of the unresolvedterthyphen;butyl proton splittings gives rise to a major contribution to the temperature variation of the linewidth in trihyphen;terthyphen;butyltropenyl. Analysis of the temperature dependence of the g value of this radical on the basis of a simple twohyphen;level scheme leads to approximate values forlpar;gEndash;gGrpar;andgG, wheregGandgEdesignate thegvalues in the ground and excited vibronic levels.