Isomérisation des radicaux insaturés. IV. Les radicaux méthyl-1-, méthyl-2- et diméthyl-l,2-propène-l yles produits dans la photolyse des tri- et tétraméthylé thylènes à 147 nm

摘要

Direct photolysis at 147 nm of tri- and tetramethylethylenes has been used to produce highly excited vinylic 1-methyl- and 2-methyl-l-propene-l-yl radicals (C4H7), as well as the 1,2-dimethyl-l-propen-l-yl radical (C5H9). In both cases, the primary fragmentation of the photoexcited molecules involved the split of an α(C-C) bond with high yields. The excess energy is sufficiently important that the majority of the vinylic radicals are very hot, and are able to isomerize towards an allylic structure before the occurrence of fragmentation. At a pressure of a few Torr, most of the vinylic C4H7 radicals (ca. 70%) isomerize to an allylic structure, and its lifetime is estimated to be less than one nanosecond under the present conditions. Although the mechanism is less simple, the excited vinylic C5H9 radical exhibits similar behaviour.