Chemical dynamics of H abstraction by OH radicals: Vibrational excitation of H2O, HOD, and D2O produced in reactions of OH and OD with HBr and DBr

Infrared chemiluminescence from vibrationally excited H2O, HOD, and D2O molecules in the ranges 3200-3900 cm(-1) (O-H stretch) and 2400-2900 cm(-1) (O-D stretch) was observed from the reactions of OH and OD radicals with hydrogen and deuterium bromide in a fast flow reactor with 0.5-2 Torr of Ar carrier gas at 300 K. Hydroxyl radicals were produced via the H + NO2 reaction; the H atoms were generated by microwave discharge in a H-2/Ar mixture. Vibrational distributions for H2O, HOD, and D2O were determined by computer simulation of the experimental emission spectra. The H2O emission from OH + HBr reaction shows inverted populations for both the collisionally coupled stretching modes and the bending mode. Inversion in the bending distribution with a maximum for v(2) = 1 is more apparent in the v(1.3) = 1 level, which is populated up to the thermochemical limit of v(2) = 5. The HOD emission from OD + HBr shows an inverted population in the O-H stretching mode with a maximum for v(3) = 2 and shows a decreasing population in the collisionally mixed O-D stretching/bending v(1,2) levels with half the molecules in the v(1) = 0 group. The distribution in v(1,2) for HOD from the OH + DBr reaction also appeared to be decreasing for v(1) > 0 levels, but collisional redistribution to v(3) = 1 seems evident from the pressure dependence of the vibrational distributions. These distributions are discussed with the aid of the information theoretic analysis and compared to F atom abstraction reactions from HBr and DBr and to quantum-scattering calculations on an OH + HBr surface. The overall vibrational energy disposal is [f(v)] approximate to 0.6, which resembles the analogous three-body cases. However, the partitioning of the energy between stretching and bending modes raises new questions about reaction dynamics.