Reactivity of HCO with CH3 and NH2 on Water Ice Surfaces. A Comprehensive Accurate Quantum Chemistry Study

Interstellar complex organic molecules (iCOMs) can be loosely defined as chemical compounds with at least six atoms in which at least one is carbon. The observations of iCOMs in star-forming regions have shown that they contain an important fraction of carbon in a molecular form, which can be used to synthesize more complexes, even biotic molecules. Hence, iCOMs are major actors in the increasing molecular complexity in space, and they might have played a role in the origin of terrestrial life. Understanding how iCOMs are formed is relevant for predicting the ultimate organic chemistry reached in the interstellar medium. One possibility is that they are synthesized on the interstellar grain icy surfaces, via recombination of previously formed radicals. The present work focuses on the reactivity of HCO with CH3/NH2 on the grain icy surfaces, investigated by means of quantum chemical simulations. The goal is to carry out a systematic study using different computational approaches and models for the icy surfaces. Specifically, DFT computations have been benchmarked with CASPT2 and CCSD(T) methods, and the ice mantles have been mimicked with cluster models of 1, 2, 18, and 33 H2O molecules, in which different reaction sites have been considered. Our results indicate that the HCO + CH3/NH2 reactions, if they actually occur, have two major competitive channels: The formation of iCOMs CH3CHO/NH2CHO or the formation of CO + CH4/NH3. These two channels are either barrierless or present relatively low (≤10 kJ/mol equal to about 1200 K) energy barriers. Finally, we briefly discuss the astrophysical implications of these findings.