Kinetics of the Methanol Reaction with OH at Interstellar, Atmospheric, and Combustion Temperatures

Abstract

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 low-temperature 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.

Rapid Acceleration of Hydrogen Atom Abstraction Reactions of OH at Very Low Temperatures through Weakly Bound Complexes and Tunneling

“…Similar behaviour has been observed for other reactions in the solid state. 23,24 There have been several theoretical studies of the OH+methanol reaction 25,26 Figure 6. 8 MESMER is able to reproduce the general shape of the upturn of k3(T) at low temperatures, but was not able to give quantitative agreement.…”

“…8 MESMER is able to reproduce the general shape of the upturn of k3(T) at low temperatures, but was not able to give quantitative agreement. A recent theoretical study also probed the shape of k3 at low temperatures, and found a significant effect of the anharmonicity of high-frequency modes of transition states, and showed that tunnelling led to an unusual negative temperature dependence below 100 K. 25 The rate of tunnelling is expected to decrease dramatically for deuterated methanol, but unfortunately rapid exchange of D with H via the walls of the gas-lines led to the formation of CD3-OH, and any so effect could not be studied experimentally. 8 The spectroscopy and dynamics of weakly-bound complexes between OH and other species have been extensively studied in free-jet expansions 28 and, more recently, following their stabilisation within helium nanodroplets.…”

Experimental Studies of Gas-Phase Reactivity in Relation to Complex Organic Molecules in Star-Forming Regions

“…[214][215][216][217] by several different groups. The reaction was first studied at low temperature by the Leeds group, who measured the rate coefficient by OH LIF at 63 K and 80 K. They found the rate coefficient for the reaction between OH and methanol was almost two orders of magnitude larger at…”

Disclosure of water roles in gliding arc plasma reforming of methanol for hydrogen production

“…The formed • H and • OH radicals may react with methanol molecules by the following reactions [43][44][45][46][47][48]…”