Unimolecular Decomposition of Chemically Activated sec-Butyl and sec-Butyl-d1 Radicals. The Reaction of H and D Atoms with trans-Butene-2 and Butene-1

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Abstract
H and D atoms were reacted with trans-butene-2 to produce vibrationally excited sec-butyl and sec-butyl-d1 radicals with a range of excess energies with respect to C–C rupture. Previous work [R. E. Harrington, B. S. Rabinovitch, and H. M. Frey, J. Chem. Phys. 33, 1271 (1960)] on butene-1 has also been extended. Observed rate constants for the unimolecular decomposition of these sec-butyl radicals relative to collisional stabilization were obtained as a function of pressure. Possible competing side reactions are discussed and evaluated. Butenyl radical formed by H abstraction from butene-2 tends to retain the geometric configuration of the original butene; also, the ratio of the pentenes formed by methyl coupling, illustrative of the two principal resonance structures, is (3-methyl-butene-1) / (total pentene-2)∼0.4. An expected energy isotope effect for D atoms, which give sec-butyl-d1 radicals having ∼1.9 kcal/mole more vibrational energy than the corresponding sec-butyl radical formed from H atoms, was seen. With butene-1 or butene-2 reactant, no evidence was found for H-atom migration in the excited sec-butyl radical formed. The disproportionation/recombination ratio of sec-butyl radicals was found from all systems to be 0.63.

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