The dissociation energies and mechanism of energy-selected bromo- and iodo-butanes

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Accurate dissociation onsets for the loss of a halogen atom from the isomers of bromo and iodobutanes have been measured from breakdown graphs and photoionization efficiency curves. The breakdown curves for 1-halobutanes, 2-halobutanes, isobutyl halides, and t-butyl halides were obtained by state-selecting the ions with the photoelectron photoion coincidence (PEPICO) technique. A comparison of the dissociation onsets with the known dissociation limits to the C 4H + 9 + X (X = Br, I) products demonstrates that only the 2-halobutanes and t-butyl halides produce the expected C 4H + 9 fragments, the 2-butyl and t-butyl ions, respectively. The 1-halobutane ion probably forms the 2-C 4H + 9 product ion, while the isobutyl halide ion dissociates to the t-butyl ion. The necessary rearrangements can be accomplished by H atom scrambling. Although the dissociations proceed via a reverse activation barrier in all cases except for the 2-butyl halide, the kinetic energy released in the reactions is small and nearly statistical. One exception is the dissociation from the excited electronic states of the t-butyl halides, which results in a large and non-statistical kinetic energy release. Dissociation rate studies of the C 4H + 9 products demonstrate that all of these ions isomerize to the t-butyl structure prior to producing C 3H + 5 + CH 4.