Paикaльныe пapы в oблyчeннoм димeтилглoкcимe: Изoтoпныe и тyннeльныe эффeкты в пpeвpaщeнияx пap

Abstract

The structure, and kinetics of transformation, of radical pairs are studied by E.S.R. in γ or electron beam irradiated single crystals of normal dimethylglyoxime (DMG) or deuterium substituted DMG: (CH 3NOD) 2. Besides isolated N-oxide radicals and radical pairs with interspin distance 5·6 Å already observed earlier in X-irradiated DMG, at least five types of radical pairs with different localizations in the crystalline cell and interspin distances 4·9, 4·6, 4·5, 3·9, and 3·2 Å have been studied. In both DMG and DMG- d 2 isolated radicals, and pairs with r av = 5·4 A ̊ , are generated with a radiation yield G ≈ 1 per 100 eV, the G-value for the other pairs being smaller by an order of magnitude. The mechanism of pairwise trapping of radicals during irradiation is briefly discussed. The transformations of radical pairs occurring between 77 and 310 K reflect processes of intra- and intermolecular free valence migration. The experimental rates show large isotope effects at low temperatures; the rate sonstants for intramolecular transfer are: 1·5 exp[ −(1400±70) RT ] s −1 (77–110 K) and 6·0 × 10 4 exp[ −(7300±700) RT ] s −1 (190–220 K) , and for intermolecular reactions 3 × 10 10 exp[ −(17,700±700) RT ] s −1 (270–310 K) and 1·7 × 10 13 exp[ −(24,200±400) RT ] s −1 (310–340 K) . for DMG and DMG - d 2 correspondingly. The fast low temperature migration and large kinetic isotope effect are believed to be connected with hydrogen tunnelling of the type: [R:NOH ON:R] → [R:NO HON:R]. At higher temperatures the radical pairs decay; rate constants for DMG and DMG - d 2 are respectively: 6·3 × 10 10 exp[ −(21,500±900) RT ] s −1 (300–340 K) and 2 × 10 11 exp[ −(22,100±900) RT ] s −1 (300–340 K) . Relative quantum yields for photoinduced reactions of radical pairs have been determined at 77 K. The observed reactions are most probably due to intra- and intermolecular hydrogen transfer induced by photoexcited radicals.