Excited radicals in the gas phase photolysis of butene isomers at different photon energies: energy distribution in reaction products

Abstract

A systematic study of the pressure effects on the quantum yields of some products between 0.1 and 600 Torr (13 and 80 000 N m−2) was carried out in the 7.6 and 8.4 eV photolysis of normal, iso- and cis-2-butenes. The propylene quantum yield (s-C4H9* → C3H6 + CH3) decreased with the increase in the n-butene pressure and a good linearity of S/D (stabilization/decomposition) vs. pressure plot, over a broad pressure region, was observed. It is concluded that hydrogen atoms involved in the s-C4H9* radical formation are produced with a relatively narrow energy distribution. The slope of S/D vs. pressure lines decreased with the increase in photon energy, indicating the trend in the kinetic energy of the H-atoms.In the case of isobutene and cis-2-butene photolysis, the Stern–Volmer plots for allene formation were nonlinear. It is concluded that the formation of two different allene precursors is needed to account for this result. By the use of a simple RRK-type formalism we also conclude that the excess energy of the photon in the primary photoexcited butene molecules is far from being randomized before their fragmentation occurs.[Formula: see text]

The fate of excited unsaturated radicals produced in the vacuumuv photolysis of gaseous olefins

“…35 Assuming that the ~(C--H)/a(C--C) ratio obtained above also applies to the 8.4 eV photolysis of isobutene, this would indicate that two main channels are available for the fragmentation of the photoexcited species: (i)/3(C--H) split and (ii) a(C--C) split. The energy required for such processes, 358 + 5 ~1 and 375 + 10 8x kJ.mol -x, respectively, are in agreement with the order of importance of the two splits.…”

“…Finally, this laboratory has recently observed anomalies in the rate constants measured at 7.1, 7.6, and 8.4 eV (35). It is thus possible that there is no continuity in the fragmentation pattern of the photoexcited isobutene molecule between 7.6 and 8.4 eV.…”