Low-Temperature Kinetic Isotope Effects in CH3OH+H -> CH2OH+H2 Shed Light on the Deuteration of Methanol in Space

TL;DR

This study uses advanced quantum chemical calculations to analyze low-temperature kinetic isotope effects in methanol reactions, providing insights into deuteration processes relevant to space chemistry.

Contribution

It introduces a neural network-fitted potential energy surface combined with instanton theory to accurately compute isotope effects at very low temperatures.

Findings

Kinetic isotope effects vary significantly at 30 K, from 4100 to 2-6.

The $^{12}$C/$^{13}$C kinetic isotope effect is 1.08 at 30 K.

Predicted high abundances of deuterated methanol in space environments.

Abstract

We calculated reaction rate constants including atom tunneling for the hydrogen abstraction reaction CH3OH+H -> CH2OH+H2 with the instanton method. The potential energy was fitted by a neural network, that was trained to UCCSD(T)-F12/VTZ-F12 data. Bimolecular gas-phase rate constants were calculated using microcanonic instanton theory. All H/D isotope patterns on the CH3 group and the incoming H atom are studied. Unimolecular reaction rate constants, representing the reaction on a surface, down to 30 K, are presented for all isotope patterns. At 30 K they range from 4100 for the replacement of the abstracted H by D to ~ 8 for the replacement of the abstracting H to about 2--6 for secondary KIEs. The $^\text{12}$C/$^\text{13}$C kinetic isotope effect is 1.08 at 30 K, while the $^\text{16}$O/$^\text{18}$O kinetic isotope effect is vanishingly small. A simple kinetic surface model using these data predicts high abundances of the deuterated forms of methanol.