The temperature dependence of the rate coefficient for the
atmospherically important radical−molecule reaction
OH + HBr has been investigated between 76 and 242 K using a pulsed
uniform supersonic flow reactor.
The current work indicates that the rate coefficient shows
significant inverse temperature dependence only
below 150 K. These results verify that within the terrestrial
atmosphere, the OH + HBr reaction manifests
a temperature-independent bimolecular rate coefficient k =
(1.2 ± 0.2) × 10-11 cm3
s-1.
The temperature dependence of the hydrogen transfer rate coefficients for the reactions: OH + HBr (Reaction 1), 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.
The temperature dependence of the rate coefficient for the reaction, OH + HBr has been reinvestigated at low temperatures (T = 48-224 K) by using uniform supersonic flow reactors with laser induced fluorescence detection. This paper presents two forms of global fits: k(T) = 1.11 × 10 −11 (T/298) −0.91 cm 3 s −1 and k(T) = 1.06 × 10 −11 (T/298) −1.09 exp( −10.5 K T ) cm 3 s −1 , both of which accurately describe the temperature dependence of the rate coefficient for the title reaction within the temperature range 20-350 K. These fits indicate that at temperatures below 200 K, the rate coefficient for this reaction shows inverse temperature dependence, while above 200 K the reaction shows insignificant temperature dependence.
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