Competitive Unimolecular Decomposition in Chemically Activated Radical Systems. Isopropyl Radical Rupture

The competitive unimolecular decomposition of vibrationally excited 5-methyl-hexyl-3· (5MH3·*) and 3,4-dimethyl-pentyl-2· (34DMP2·*) radicals was studied in chemical activation systems in which H atoms were added to suitable olefins. The relative rate of isopropyl versus methyl-radical rupture from each radical yields D0°(iPr–H) = 92.8 kcal mole−1. Absolute rates of decomposition of 5MH3·*, 2MH3·*, and 34DMP2·* are compared to values obtained from the theoretical model and show good agreement The reactions of isopropyl radicals with 5MH3·, 2MH3·, and 34DMP2· radicals are discussed briefly; disproportionation-recombination ratios are obtained and compare favorably with values calculated according to an empirical prescription given previously. The measurement of the relative rates of decomposition of 5MH3·* and of 2MH3·* in the pseudocompetitive reaction system, H+trans-2-methylhexene-3, provides a relation between the energetics of the 5MH3· radical and the 2MH3· radical and supports D0°(5MH3·–H) = D0°(2MH3·–H). A summary of bond dissociation energies determined by the present method is given.