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The anisotropic hyperfine splitting due to C13 in the methyl position of triphenylmethyl has been obtained from dilute Analysis of the measurements yields an estimate of density of unpaired The A simple valence bond approximation is in good solutions of triphenylmethyl in triphenylamine. electron a t the methyl position. magnitudes of the densities require that some of them have negative sign.Estimates of densities a t other positions are made from proton hyperfine splittings. agreement with the observations.

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The e.p.r. spectrum of thiyl radicals is obtained when dilute solutions of aliphatic mercaptans are photolyzed in a xenon matrix at 77 OK. No proton hyperfine structure is observed at low concentrations of thiol but it appears at higher concentrations due to the reaction of hot hydrogen atoms with solute mercaptan molecules. When small amounts of the thiols are dissolved in glassy hydrocarbon matrices such as 3-methylpentane, there is produced after photolysis hyperfine structure characteristic of the solvent as well as the thiyl resonance. The e.s.r. absorption due to the solvent species disappears when the matrix is warmed a few degrees above 77 "K but thiyl radicals are stable up to the melting point of the glass. The corresponding y-irradiated glasses do not give thiyl absorption, but two other species are produced along with solvent radicals. No thiyl radicals are observed following radiolysis, or upon photobleaching the new species. The spatial distribution of the neutral radicals in the glasses is discussed in view of these results.Le spectre de r.p.e. de radicaux thiyl est obtenu lors de la photolyse de solutions dilukes de mercaptans aliphatiques dans une matrice de xenon a 77 OK. On n'observe pas la structure hyperfine du proton en utilisant de faibles concentrations de thiol, mais elle apparait lorsqu'on utilise des concentrations plus ClevCes a cause de la reaction des atomes d'hydrogkne chauds avec les molCcules de mercaptan en solution. Si on dissout de petites quantites de thiol dans des matrices d'hydrocarbure vitreuses, tel le methylpentane-3, il se produit apres photolyse, une structure hyperfine qui est caracteristique du solvant aussi bien que de la resonance du thiyl. L'absorption r.p.e. due a l'espece solvant disparait quand la matrice est chauffee a quelques degris au-dessus de 77 OK, mais les radicaux thiyl demeurent stables jusqu'au point de fusion du verre. Les verres irradits aux rayons-y ne donnent pas d'absorption thiyl, mais deux autres especes sont produites en meme temps que les radicaux de solvant. On n'observe pas de radicaux thiyl apres radiolyse, ou pendant la photodCcoloration des nouvelles espkces. On discute de la distribution spatiale des radicaux neutres dans les verres pour arriver aces rksultats.

Dilute solutions of aliphatic thiols have been prepared and photolyzed at 253.7 nm in 3-methylpentane glasses at 77°K. The major products formed initially are thiyl radicals RS• and hydrogen atoms. Both of these are hot and yield solvent radicals by abstraction reactions, and relatively small amounts of RS2•, RS•, and possibly R1R2C=S. Thermal annealing of the matrices is accompanied by loss of hydrocarbon radicals through cage reactions and concommitant increase in the RS• concentration. An intermediate radical of the type R1R2ĊSH may also be produced in the annealing process. The photometric and spin resonance characteristics of RS•, RS2•and R1ĊH—SH are discussed and related to other systems in which these intermediates are observed. RS• is characterized in the amorphous phase by an absorption at 405 nm and a broad asymmetrical e.p.r. resonance near [Formula: see text] RS2• has a non-axial g-tensor with principal values of about 2.06, 2.02, and 2.00 and has no detectable absorptions between 350 and 1200 nm.The photolytic behavior of these metastable radical species is also investigated.