Gliding on ice in search of accurate and cost-effective computational methods for astrochemistry on grains: the puzzling case of the HCN isomerization

TL;DR

This study evaluates computational methods for modeling HCN to HNC isomerization on icy grains, revealing a proton relay mechanism that lowers activation energy and facilitates isomerization even at low temperatures.

Contribution

It introduces a combined QM/QM'/MM approach with large water clusters to accurately model surface effects on isomerization energy barriers.

Findings

Proton relay involving four water molecules reduces activation energy.

Large water clusters have negligible impact on the energy barrier.

Isomerization could occur easily at low temperatures due to tunneling.

Abstract

The isomerization of hydrogen cyanide to hydrogen isocyanide on icy grain surfaces is investigated by an accurate composite method (jun-Cheap) rooted in the coupled cluster ansatz and by density functional approaches. After benchmarking density functional predictions of both geometries and reaction energies against jun-Cheap results for the relatively small model system HCN -- (H2O)2 the best performing DFT methods are selected. A large cluster containing 20 water molecules is then employed within a QM/QM$'$ approach to include a realistic environment mimicking the surface of icy grains. Our results indicate that four water molecules are directly involved in a proton relay mechanism, which strongly reduces the activation energy with respect to the direct hydrogen transfer occurring in the isolated molecule. Further extension of the size of the cluster up to 192 water molecules in the framework of a three-layer QM/QM'/MM model has a negligible effect on the energy barrier ruling the isomerization. Computation of reaction rates by transition state theory indicates that on icy surfaces the isomerization of HNC to HCN could occur quite easily even at low temperatures thanks to the reduced activation energy that can be effectively overcome by tunneling.