EVALUATION OF CENTRAL AND TERMINAL ALPHA( C-H) BOND-CLEAVAGE IN THE PHOTOLYSIS OF PROPENE GAS AT 184.9 NM

The minor, alpha(C-H) photofragmentation processes in the far-UV photo lysis of gaseous propene at lambda = 184.9 nm are investigated. At thi s wavelength the beta(C-H) and alpha(C-C) photofragmentation processes are of major importance, accounting for more than 95% of the primary reaction process. The total quantum yield of the three alpha(C-H) bond cleavage processes is close to 0.03. However phi(H secondary) = 0.02 +/- 0.01 and the sum of the two phi(H primary) cleavages is 0.005 +/- 0.003. Thus phi(H secondary)/phi(H primary) congruent to 4.1 whereas s tatistically one expects 1:2. Thus the energy of the bond that is brea king is important, even though the photon energy is much higher than t hat required to break it. The geometrical parameters, total energies, and fundamental frequencies of the cis- and trans-propen-1-yl and prop en-2-yl radicals have been determined by ab initio methods at various levels of theory (STO-3G, 3-21G, 6-31G, 6-31G, and 6-31G** basis sets ; UHF, ROI-IF, and GVB methods). cis-Propen-1-yl is predicted to be 0. 6-1.0 kJ mol(-1) lower in energy than transpropen-1-yl, except with th e minimal STO-3G basis set. The optimizations predict that propen-2-yl is more stable than the cis- and trans-propen-1-yl radicals by 14 +/- 3 kJ mol(-1).