The OH radical is the most important radical in combustion and in the atmosphere, and methanol is a fuel and antifreeze additive, model biofuel, and trace atmospheric constituent. These reagents are also present in interstellar space. Here we calculate the rate constants, branching ratios, and kinetic isotope effects (KIEs) of the hydrogen abstraction reaction of methanol by OH radical in a broad temperature range of 30-2000 K, covering interstellar space, the atmosphere, and combustion by using the competitive canonical unified statistical (CCUS) model in both the low-pressure and high-pressure limits and, for comparison, the pre-equilibrium model. Coupled cluster CCSD(T)-F12a theory and multi-reference CASPT2 theory were used to carry out benchmark calculations of the stationary points on the potential energy surface to select the most appropriate density functional method for direct dynamics calculations of rate constants. We find a significant effect of the anharmonicity of high-frequency modes of transition states on the lowerature rate constant, and we show how tunneling leads to an unusual negative temperature dependence of the rate constants in the range 200 K > T > 100 K. The calculations also demonstrate the importance of the extent of stabilization of the pre-reactive complex. The capture rate for the formation of the complex is the dominant dynamical bottleneck for T < 100 K, and it leads to weak temperature dependence of the rate below 100 K in the high-pressure-limit of the CCUS model. We also report the pressure dependence of branching ratios (which are hard to measure so theory is essential) and the KIEs, and we report an unusual nonmonotonic variation of the KIE in the high-pressure limit at low temperatures.
Bibliographical noteFunding Information:
The authors are grateful to Junwei Lucas Bao and Steven L. Mielke for helpful assistance and to M. C. Lin for helpful correspondence. This work was supported in part by the National Natural Science Foundation of China (91641127), the U.S. Department of Energy under Award Number DESC0015997, the Ministerio de Economia y Competitividad of Spain (Research Grant No CTQ2014-58617-R), the Con-sellerıá de Cultura, Educacioń e OrdenaciońUniversitaria (Centro singular de investigacioń de Galicia acreditacioń 2016-2019, ED431G/09), and the European Regional Development Fund (ERDF).
© 2018 American Chemical Society.