Primary processes in the photolysis of propane: the use of HI as a radical scavenger

Abstract

The photolysis of C 3D 8 has been investigated with 8.4, 10.0, and 11.6–11.8 eV photons, using HI to scavenge the radicals through the reaction: R D + HI → R DH + I (where R D is a fully deuterated alkyl or alkenyl radical). Comparison of the results with the results of analogous experiments using H 2S as a scavenger leads to the conclusion that HI is a more efficient radical scavenger than H 2S. The results are discussed with particular emphasis on determining whether the primary processes include direct C C and C H bond cleavage. An examination of the effects of HI concentration, conversion, and pressure on the yield of ethyl radicals intercepted indicates that the ethyl radicals are formed in the primary process: C 3D 8* → C 2D 5 + CD 3. It is noted that the relative importances of this process and the other primary processes involving breaking of the C C bond (C 3D 8* → CD 4 + C 2D 4 and C 3D 8* → C 2D 6 + CD 2) do not change with energy, and it is thus suggested that they all occur from an excitation in the C C bond, and that RRKM considerations relating to equipartition of energy are not applicable to the dissociations of electronically excited alkanes. It is pointed out that, as in the photolysis of ethane, the dissociation leading to the formation of a molecule of hydrogen (deuterium) (C 3D 8* → D 2 + C 3D 6), which apparently occurs as a result of an excitation in a C H (C D) bond, predominates in the 8.4 eV photolysis, but diminishes sharply in importance with respect to the C C bond cleavage processes when the energy is increased. The insertion of methylene into a primary C H bond of C 3H 8 to give n-butane is examined, and information concerning the internal energy of the CH 2 species is derived and discussed in terms of the primary dissociation of propane.