Temperature Dependence and Kinetic Isotope Effects for the OH + HBr Reaction and H/D Isotopic Variants at Low Temperatures (53−135 K) Measured Using a Pulsed Supersonic Laval Nozzle Flow Reactor

Abstract

The reactions of OH + HBr and all isotopic variants have been measured in a pulsed supersonic Laval nozzle flow reactor between 53 and 135 K, using a pulsed DC discharge to create the radical species and laser induced fluorescence on the A 2sigma <-- X 2pi (v' = 1 <-- v'' = 0) transition. All reactions are found to possess an inverse temperature dependence, in accord with previous work, and are fit to the form k = A(T/298)(-n), with k1 (OH + HBr) = (10.84 +/- 0.31) x 10(-12) (T/298)(-0.67+/-0.02) cm3/s, k2 (OD + HBr) = (6.43 +/- 2.60) x 10(-12) (T/298)(-1.19+/-0.26) cm3/s, k3 (OH + DBr) = (5.89 +/- 1.93) x 10(-12) (T/298)(-0.76+/-0.22) cm3/s, and k4 (OD + DBr) = (4.71 +/- 1.56) x 10(-12) (T/298)(-1.09+/-0.21) cm3/s. A global fit of k vs T over the temperature range 23-360 K, including the new OH + HBr data, yields kT = (1.06 +/- 0.02) x 10(-11) (T/298)(-0.90+/-0.11) cm3/s, and (0.96 +/- 0.02) x 10(-11) (T/298)(-0.90+/-0.03) exp((-2.88+/-1.82 K)/T) cm3/s, in accord with previous fits. In addition, the primary and secondary kinetic isotope effects are found to be independent of temperature within experimental error over the range investigated and take on the value of (kH/kD)(AVG) = 1.64 for the primary effect and (kH/kD)(AVG) = 0.87 for the secondary effect. These results are discussed within the context of current experimental and theoretical work.

Development of a pulsed uniform supersonic gas expansion system based on an aerodynamic chopper for gas phase reaction kinetic studies at ultra-low temperatures

“…For all these versions, pulsed electromagnetic valves have been used to introduce the carrier gas into a very small reservoir (∼1 cm 3 ) stuck to the Laval nozzle. This design and the small pumping capacities used by the different groups restrict the lowest achievable temperature to 56 K (Gómez Martín et al, 2014) without precooling and 53 K with mild precooling (Mullen and Smith, 2005). Nonetheless, lower temperatures were obtained recently in studies not directly linked to reaction kinetics.…”

Gas phase reactive collisions, experimental approach

“…The minimum temperature, however, is limited to 53 K [91,92] due to quite small pumping capacities. This record for a pulsed apparatus was recently beaten in Rennes with the development of a new apparatus [43].…”

Gas Phase Reactive Collisions at Very Low Temperature: Recent Experimental Advances and Perspectives

“…Smith and co-workers. 19 Using this apparatus they reported investigations of the OH + HBr reaction, as well as the reactions of the NH radical with NO and hydrocarbons down to 53 K. 42,43 In the late 1990s a similar apparatus was developed by the group of Leone. 20 It has been mostly used to study reactions of the C 2 H radical with a wide range of hydrocarbons for Titan chemistry.…”

“…For the clarity of and the reaction of OH/OD with HBr/DBr from 53 K to 135 K. 43 The KIE is said to be "primary" when the isotopic replacement is involved in a chemical bond that is broken or formed and "secondary" when the substitution is not directly involved in the formed or broken bond. The KIE is usually measured as the ratio of the rate coefficient for the unsubstituted molecules (k H ) over the rate coefficient for the reaction involving the isotopically substituted molecules (k D ).…”