H2S as a Free-Radical Interceptor in the Gas-Phase Radiolysis and Photolysis of Propane

Abstract

The gas-phase photolysis (1236 Å) and the γ-ray radiolysis of C3D8 has been investigated in the presence of varying concentrations of H2S. When 10% or more H2S is added to C3D8, the majority of the D, CD3, C2D3, and C2D5 radicals abstract an H atom from H2S to form HD, CD3H, C2D3H, and C2D5H, respectively. The fully deuterated molecules formed in these mixtures result from the unimolecular elimination of a stable molecule from C3D8 or C3D8+ and from fast bimolecular processes such as ion—molecule reactions The mechanisms of the radiolysis and the photolysis proposed in earlier studies have been re-examined in the light of the information derived from the C3D8—H2S experiments and of some additional photolysis experiments on CD3CH2CD3—NO mixtures. The results indicate that the modes of decomposition of the neutral excited propane molecule are as follows: C3D8*→C2D6+CD2,→CD4+C2D4,→D2+C3D6,→CD3+C2D5,→D+C3D7.The internally excited C2D4, C2D5, C3D6, and C3D7 species formed in these primary processes decompose to form D, CD3, C2D2, C2D3, and C2D4 unless they are collisionally stabilized. In the radiolysis of C3D8—H2S mixtures, yields of the free radicals can be adequately accounted for by taking into account the modes of fragmentation of the parent ion and of the excited propane. The C2D5+ ions are shown to react with H2S in part by the deuteron-transfer reaction C2D5++H2S→C2D4+DH2S+.The effect of pressure and the effectiveness of HI as a free-radical interceptor in the radiolysis have been examined.