Kinetic studies of the reactions of atomic hydrogen with iodoalkanes

Abstract

Rate constants for the reaction of H atoms with the alkyl iodides iodomethane (1), deuterated iodomethane (1d), iodoethane (2), 2-iodopropane (3), and 2-iodo-2-methyl propane (4) have been measured using the flash-photolysis resonance fluorescence technique. The results are k1 = (6.3 ± 0.6) × 10-11 exp[((-5.0 ± 0.3) kJ mol-1)/RT] (T = 297-757 K), k1d = (8.0 ± 1.7) × 10-11 exp[((-5.3 ± 0.7) kJ mol-1)/RT] (T-296-728 K), k2 = (1.1 ± 0.2) × 10-10 exp[((-5.9 ± 0.8) kJ mol-1)/RT] (T= 295-624 K), k3 = 1.4 × 10-11 (T = 295 K) and k4 = 2.0 × 10-11 (T = 294 K) cm3 molecule-1 s-1. The transition state for the substitution reaction H + CH3I → I + CH4 was characterized at the Gaussian-2 level of ab initio theory, and the process was shown to be slow. Evidence is presented showing that H-atom abstraction is also slow, and that the dominant pathway for the reactions of H atoms with iodoalkanes is I-atom abstraction.