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

Abstract

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 1 arXiv:1909.12686v1 [astro-ph.SR] 27 Sep 2019 carbon in a molecular form, which can be used to synthesize more complex, 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 CH 3 /NH 2 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 bench-marked with CASPT2 and CCSD(T) methods, and the ice mantles have been mimicked with cluster models of 1, 2, 18 and 33 H 2 O molecules, in which different reaction sites have been considered.Our results indicate that the HCO + CH 3 /NH 2 reactions, if they actually occur, have two major competitive channels: the formation of iCOMs CH 3 CHO/NH 2 CHO, or the formation of CO + CH 4 /NH 3 . 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.

Revisiting the reactivity between HCO and CH3 on interstellar grain surfaces

“…Rubin et al (1971);Cazaux et al Kahane et al (2013); Mendoza et al (2014); López-Sepulcre et al (2015); Belloche et al (2017); Ligterink et al (2017); McGuire et al (2018)), in the circumstellar envelopes of AGB stars (Cernicharo et al (2000)), shocked regions (Arce et al (2008); Codella et al (2017); Lefloch et al (2017)) and even in external galaxies (Muller et al (2013)). Despite their presence has been known for decades, how iCOMs are synthesized is still an open question and under debate (Herbst & van Dishoeck (2009) ;Caselli & Ceccarelli (2012); Woods et al (2013); Balucani et al (2015); Butscher et al (2017); Butscher et al (2019); Fedoseev et al (2015); Enrique-Romero et al (2016); Enrique-Romero et al (2019); Gal et al (2017); Rivilla et al (2017); Vasyunin et al (2017); Rimola et al (2018); Lamberts et al (2019)). Two different paradigms have been proposed: i) on the surfaces of grains (either during the cold prestellar or warmer collapse phase (e.g.…”

“…This unexpected result called for and was followed by other studies of different systems and with different computational methods. First, Rimola et al (2018) and Enrique-Romero et al (2019) studied the formation of formamide (NH 2 CHO) and acetaldehyde by reactions between HCO and NH 2 and HCO and CH 3 on a AWS model by means of static quantum chemical calculations. Subsequently, Lamberts et al (2019) studied the formation of acetaldehyde by reaction between of HCO and CH 3 on a CO-pure ice model by means of ab initio molecular dynamics (AIMD) simulations.…”

“…In this article, we aim to revise the first calculations carried out on the CH 3 + HCO system (Enrique-Romero et al (2016)) which were based on the standard DFT approach. Since then, it has become clear that an improved treatment of the radicals spins is necessary (Rimola et al (2018); Enrique-Romero et al (2019)). The article is organised as follows: in § 2 we review the treatment of the spins of a biradical system, in § 3 we provide the details of the new computations carried out in this article, in § 4 we show the results and in § 5 we discuss the conclusions.…”

A Combined DFT and RRKM-Based Study on the Reactivity of HCO + NH$$_2$$ on Amorphous Water Ice Surface

“…A generally accepted pathway to form iCOMs like formamide is via radical-radical couplings (Rc) on or/and in ice mantles [ 10 ]. Recent works [ 8 , 15 ] investigated the formation of formamide adopting this mechanism (i.e. the radical-radical combination of NH and HCO) by means of quantum chemistry calculations and using as amorphous solid water (ASW) a water cluster model of 33 water molecules.…”

Astrochemistry as a Tool To Follow Protostellar Evolution: The Class I Stage

“…Then, at about 100 K the entire mantle sublimates injecting into the gas-phase the iCOMs formed in this way. Theoretical chemistry studies, however, show that this combination of radicals does not necessarily end up in iCOMs 61,62 . Alternatively, the mantle simple molecules (such as methanol or formaldehyde) formed during the prestellar phase are injected into the gas at 100 K and undergo reactions that form iCOMs [63][64][65] .…”