Temperature-dependent kinetic isotope effects in the gas-phase reaction: OH+HBr

The temperature dependence of the hydrogen transfer rate coefficients for the reactions: OH + HBr (Reaction I), OD + HBr (Reaction 2), OH + DBr (Reaction 3), and OD + DBr (Reaction 4) have been investigated at temperatures between 120 and 224 K using a pulsed uniform supersonic flow monitoring hydroxyl reactive loss. The lack of observed isotopic scrambling indicates the reaction occurs by H/D atom transfer from HBr/ DBr at all temperatures. The rate coefficients demonstrate little temperature dependence above 200 K, but strong inverse temperature behavior below 200 K. The current work provides unequivocal experimental evidence of temperature dependent and inverse primary and secondary kinetic isotope effects (k(H)/k(D) < 1) at low temperatures. The observed kinetic isotope ratios, k(H)/k(D), at 120 K are for primary substitution on HBr; k(1)/k(3) = 1.00 (+/-0.17) and k(2)/k(4) = 0.46 (+/-0.08) while for secondary substitution on OH; k(1)/k(2) = 0.94 (+/-0.20) and k(3)/k(4) = 0.43 (+/-0.05). At the lowest temperature employed (120 K), deuterated reactants react as fast or faster than their natural hydrogen isotopomer and there is no significant difference between the primary and secondary kinetic isotope effect. The results are discussed within the framework of recent theoretical models.