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)CN^˙. 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.