Cracking the puzzle of CO₂ formation on interstellar ices

Quantum chemical and kinetic study of the CO + OH → CO₂ + H reaction

¹ Department of Astronomy, Graduate School of Science, The University of Tokyo, Tokyo 113 0033, Japan
e-mail: molpeceres@astron.s.u-tokyo.ac.jp
² Leiden Institute of Chemistry, Gorlaeus Laboratories, Leiden University, PO Box 9502, 2300 RA Leiden, The Netherlands
e-mail: j.enrique.romero@lic.leidenuniv.nl

Received: 5 June 2023
Accepted: 6 July 2023

Abstract

Context. Carbon dioxide (CO₂) is one of the dominant components of interstellar ices. Recent observations show CO₂ exists more abundantly in polar (H₂O-dominated) ice than in apolar (H₂O-poor) ice. Formation of CO₂ ice is primarily attributed to the reaction between CO and OH, which has a barrier.

Aims. We investigate the title reaction in H₂O ice and CO ice to quantify the efficiency of the reaction in polar ice and apolar ice.

Methods. 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 H₂O and CO clusters. Microcanonical transition state theory was used as a diagnostic tool for the efficiency of the reaction under interstellar medium conditions. We simulated the kinetics of ice chemistry, considering different scenarios involving non-thermal processes and energy dissipation.

Results. 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 an H₂O cluster or a CO cluster.

Conclusions. 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 using low-temperature OH radicals is encouraged.