Cracking the Puzzle of CO2 Formation on Interstellar Ices. Quantum Chemical and Kinetic Study of the CO + OH -> CO2 + H Reaction

TL;DR

This study uses quantum chemical calculations and kinetic modeling to investigate the CO + OH reaction's role in interstellar CO2 ice formation, revealing it alone cannot account for observed abundances, especially in apolar ice.

Contribution

The paper provides a detailed quantum chemical and kinetic analysis of the CO + OH reaction, highlighting its limited role in CO2 formation on interstellar ices under various conditions.

Findings

CO + OH reaction proceeds via HOCO intermediate.

HOCO formation is efficient on H2O clusters but not on CO clusters.

CO2 formation via this pathway is unlikely without external energy.

Abstract

CO2 is one of the dominant components of the interstellar ice. Recent observations show CO2 exists more abundantly in polar (H2O-dominated) ice than in apolar (H2O-poor) ice. CO2 ice formation is primarily attributed to the reaction between CO and OH, which has a barrier. Highly accurate quantum chemical calculations were employed to analyze the stationary points of the potential energy surfaces of the title reaction in the gas phase on a H2O and CO clusters. Microcanonical transition state theory was used as a diagnostic tool for the efficiency of the reaction under ISM conditions. We simulate the kinetics of ice chemistry, considering different scenarios involving non-thermal processes and energy dissipation. The CO + OH reaction proceeds through the remarkably stable intermediate HOCO radical. On the H2O cluster, the formation of this intermediate is efficient, but the subsequent reaction leading to CO2 formation is not. Conversely, HOCO formation on the CO cluster is inefficient without external energy input. Thus, CO2 ice cannot be formed by the title reaction alone either on the H2O cluster or CO cluster. In the polar ice, CO2 ice formation is possible via CO + OH -> HOCO, followed by HOCO + H ->CO2 + H2, as demonstrated by abundant experimental literature. In apolar ice, CO2 formation is less efficient because HOCO formation requires external energy. Our finding is consistent with the JWST observations. Further experimental work is encouraged using low-temperature OH radicals.