Low Temperature Kinetics of the CH₃OH + OH Reaction

Abstract

The rate constant of the reaction between methanol and the hydroxyl radical has been studied in the temperature range 56-202 K by pulsed laser photolysis-laser induced fluorescence in two separate experiments using either a low temperature flow tube coupled to a time of flight mass spectrometer or a pulsed Laval nozzle apparatus. The two independent techniques yield rate constants which are in mutual agreement and consistent with the results reported previously below 82 K [Shannon et al., Nature Chemistry, 2013, 5, 745-749] and above 210 K [Dillon et al., Phys. Chem. Chem. 2005, 7, 349-355], showing a very sharp increase with decreasing temperature with an onset around 180 K. This onset is also signalled by strong chemiluminescence tentatively assigned to formaldehyde, which is consistent with the formation of the methoxy radical at low temperature by quantum tunnelling, and its subsequent reaction with H and OH. Our results add confidence to the previous low temperature rate constant measurements and consolidate the experimental reference dataset for further theoretical work required to describe quantitatively the tunnelling mechanism operating in this reaction. An additional measurement of the rate constant at 56 K yielded a value of (4.9 ± 0.8) × 10 −11 cm 3 molecule −1 s −1 (2σ), showing that the rate constant is increasing less rapidly at temperatures below 70 K.

The origin of gas-phase HCO and CH3O radicals in prestellar cores

“…Those results were confirmed and extended to lower temperatures by Gómez Martín et al (2014). Similarly, other neutral-neutral reactions, such as the barrierless reaction CH 3 OH + CN (Sayah et al 1988), also produce CH 3 O. HCO can be formed with the corresponding reactions involving H 2 CO instead of CH 3 OH and also with CH 3 OH + C (Shannon et al 2014).…”

Measurements of Rate Coefficients for Reactions of OH with Ethanol and Propan-2-ol at Very Low Temperatures

“…14,17,22 Furthermore, the formation of the longer-lived complex at low temperature enhances the probability of hydrogen atom transfer via quantum mechanical tunneling to form products. It is postulated that this mechanism may be general for reactions of OH with oxygenated volatile compounds, and that these reactions should be considered in networks describing the chemistry of the interstellar medium.…”

“…17,22 At low temperatures, the reaction leads to the formation of water vapor and the methoxy radical (CH 3 O) as products. 17 The increase in the rate coefficient is attributed to quantum mechanical tunneling through the barrier to hydrogen abstraction, a mechanism which has also been observed in the reactions of OH with dimethyl ether and acetone, 15 and is expected to be applicable to reactions of OH with other oxygenated VOC reactions at very low temperatures.…”

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.…”