Electron spin resonance study of azoalkane and imine radical cations

Abstract

Radical cations of azoalkanes have been observed for the first time in a low-temperature matrix by e.s.r. spectroscopy. The e.s.r. data show that they are π-cations despite photoelectron studies which indicate the HOMO for an azoalkane to be the σ(n_-) orbital. Interestingly, these species appear to be stable, at least up to ca. 160 K, and show no tendency to fragment to form alkyl radicals. This contrasts with the solution-phase behaviour of these species, which are intermediates in the oxidation of azoalkanes, and give rise to carbocations and alkyl radicals. Imines form similar π-radical cations which show a greater tendency to decompose. The structures and stabilities of these cations are discussed. Attempts to observe the corresponding azoalkane anions in solid matrices resulted only in the formation of alkyl radicals, and so we conclude that the anions decompose by fragmentation, although they have been studied in solution under steady-state conditions. Previously derived coupling parameters predict isotropic ¹⁴N hyperfine couplings for these species which are larger than those observed. Therefore, new values are calculated, and these fit well with the observed couplings. In this way, spin polarisation parameters are obtained for individual contributions to the couplings, as opposed to previous approaches which treat them collectively.