Electron-addition and electron-loss pathways for cyanoalkanes
Abstract
Three systems involving cyanomethane, cyanoethane, 2-cyanopropene 2-cyano-2-methylpropane and 1,1-dicyanoethane have been exposed to 60Co γ-rays at 77 K, and the resulting radicals studied using e.s.r. spectroscopy. Using solvent CD3OD, we found that the expected radical anions, (RCN)˙– reacted promptly at 77 K by two independent paths, one giving R radicals in their normal form together, presumably, with solvated CN–, the other giving R(D)CN˙ or, using CD3OH, R(H)C
N˙. The latter radicals are characterised by a large proton hyperfine coupling in the 80–85 G range. In contrast, 1,2-dicyanoethane gave a centre with maximum coupling to two 14N nuclei of 17 G, thought to be the parent radical anion with a cyclic structure analogous to the well known cyanomethane dimer anion, (MeCN)2˙. Solutions in CFCl3 failed to give primary cations except for Me3CCN which gave a solvent adduct, Me3CN—C(CFCl2+, characterised by large hyperfine coupling to 14N and 35Cl. On annealing this species was apparently converted into Me3C˙ radicals. On exposure to visible light the adduct gave a radical cation tentatively identified as (H2CĊMe2)+. Mechanisms for these reactions are considered. The pure compounds have also been studied as powders. Results for CH3CN and CD3CN were in agreement with those previously published. Cyanoethane gave an extremely complicated spectrum, but features for CH3CHCN, EtC(H)
N˙ and a modified form of CH3CH2˙ radicals with reduced proton coupling were identified. Probably the ethyl radicals form a weak complex with CN–. Similar results were obtained for Me2CHCN. For Me3CCN, well resolved features for Me3C˙ and Me3CC(H)
N˙ radicals were obtained. On annealing, the former radicals were lost, and features asigned to H2ĊC(Me)2CN radicals grew in. Both these radicals and the hydrogen adduct gave isotropic e.s.r. spectra at ca. 150 K.