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

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 CH$_3$ 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 12C/13C kinetic isotope effect is 1.08 at 30~K, while the 16O/18O kinetic isotope effect is vanishingly small. A simple kinetic surface model using these data predicts high abundances of the deuterated forms of methanol.