Low-Temperature Kinetic Isotope Effects in CH 3 OH + H → CH 2 OH + H 2 Shed Light on the Deuteration of Methanol in Space

We calculated reaction rate constants including atom tunneling for the hydrogen abstraction reaction CH 3 OH + H → CH 2 OH + H 2 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 ∼2 to 6 for secondary KIEs. The 12 C/ 13 C kinetic isotope effect is 1.08 at 30 K, while the 16 O/ 18 O kinetic isotope effect is extremely small. A simple kinetic surface model using these data predicts high abundances of the deuterated forms of methanol.