Branching Ratios and Vibrational Distributions in Water-Forming Reactions of OH and OD Radicals with Methylamines

Reactions of OH and OD radicals with (CH3)(3)N, (CH3)(2)NH, and CH3NH2 were studied by Fourier transform infrared emission spectroscopy (FTIR) of the water product molecules from a fast-flow reactor at 298 K. The rate constants (4.4 +/- 0.5) x (5.2 +/- 0.8) x 10(-11), and (2.0 +/- 0.4) x 10(-11) cm(3) molecule(-1) s(-1) were determined for OD + (CH3)(3)N, (CH3)(2)NH, and CH3NH2 respectively, by comparing the HOD emission intensities to the HOD intensity from the OD reaction with H2S. Abstraction from the nitrogen site competes with abstraction from the methyl group, as obtained from an analysis of the HOD and D2O emission intensities from the OD reactions with the deuterated reactants, (CD3)(2)NH and CD3NH2. After adjustment for the hydrogen deuterium kinetic isotope effect, the product branching fractions of the hydrogen abstraction from the nitrogen for di- and monomethylamine were found to be 0.34 +/- 0.04 and 0.26 +/- 0.05, respectively. Vibrational distributions of the H2O, HOD, and D2O molecules are typical for direct hydrogen atom abstraction from polar molecules, even though activation energies are negative because of the formation of pre-transition-state complexes. Comparison is made to the reactions of hydroxyl radicals with ammonia and with other compounds with primary C-H bonds to discuss specific features of disposal of energy to water product.