Cracking the puzzle of CO₂ formation on interstellar ices

Context . Carbon dioxide (CO 2 ) is one of the dominant components of interstellar ices. Recent observations show CO 2 exists more abundantly in polar (H 2 O-dominated) ice than in apolar (H 2 O-poor) ice. Formation of CO 2 ice is primarily attributed to the reaction between CO and OH, which has a barrier. Aims . We investigate the title reaction in H 2 O 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 H 2 O 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 H 2 O cluster, the formation of this intermediate is efficient, but the subsequent reaction leading to CO 2 formation is not. Conversely, HOCO formation on the CO cluster is inefficient without external energy input. Thus, CO 2 ice cannot be formed by the title reaction alone either on an H 2 O cluster or a CO cluster. Conclusions . In the polar ice, CO 2 ice formation is possible via CO + OH → HOCO followed by HOCO + H → CO 2 + H 2 , as demonstrated by abundant experimental literature. In apolar ice, CO 2 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.