Combined quantum chemical and modeling study of CO hydrogenation on water ice

Abstract

Context. Successive hydrogenation reactions of CO on interstellar icy grain surfaces are considered one of the most efficient mechanisms in interstellar environments for the formation of H 2 CO and CH 3 OH, two of the simplest organic molecules detected in space. In the past years, several experimental and theoretical works have been focused on these reactions, providing relevant information both at the macroscopic and atomic scale. However, several questions still remain open, such as the exact role played by water in these processes, a crucial aspect because water is the dominant constituent of the ice mantles around dust grain cores. Aims. We here present a quantum chemical description of the successive H additions to CO both in the gas phase and on the surfaces of several water clusters. Methods. The hydrogenation steps were calculated by means of accurate quantum chemical methods and structural cluster models consisting of 3, 18, and 32 water molecules. Results. Our main result is that the interaction of CO and H 2 CO with the water cluster surfaces through H-bonds with the O atoms increases the C−O polarization, thus weakening the C−O bond. Consequently, the C atoms are more prone to receiving H atoms, which in turn lowers the energy barriers for the H additions compared to the gas-phase processes. The calculated energy barriers and transition frequencies associated with the reaction coordinate were adopted as input parameters in our numerical model of the surface chemistry (GRAINOBLE) to simulate the distribution of the H 2 CO and CH 3 OH ice abundances (with respect to water). Our GRAINOBLE results based on the energy barriers and transition frequencies for the reactions on the 32 water molecule cluster compare well with the observed abundances in low-mass protostars and dark cores.

L1157-B1, a factory of complex organic molecules in a solar-type star-forming region

“…Cuppen et al 2009;Rimola et al 2014). The correlations methanol-ethanol and ethanol-glycolaldehyde might then suggest that ethanol and glycolaldehyde are formed on the grain surfaces, as suggested by some experiments.…”

Grain Surface Models and Data for Astrochemistry

“…Surface reactions require many atoms in the calculation, which unequivocally increases the computational time. One of the approximations that can be made is the use of a model surface, e.g., a coronene molecule to model a carbonaceous grain surface (Adriaens et al 2010), or a water cluster to mimic a thick ice layer (Rimola et al 2014). Another promising method that can be used to include the effect of the surface on reaction rates is quantum mechanics/molecular mechanics (QM/MM, Goumans et al 2009).…”

Dimerization of methanimine and its charged species in the atmosphere of Titan and interstellar/cometary ice analogs

“…We can conclude, therefore, that polymerization of methanimine in the gas-phase at low temperatures may well be initiated by the presence of an ionized molecule. As for the experiment on ice by Bernstein et al (1995), it is well known that reaction barriers possibly present in gas-phase reactions are not significantly reduced when moving to ice-mediated reactions (see, for instance, Rimola et al 2014). Normally, the tunneling effect is invoked to explain the observed reactivity, but in this case the reaction barriers are so high that it is difficult to think that a reaction sequence starting with dimerization of neutral methanimine molecules can account for the observed formation of hexamethylenetetramine or polymethylenimine.…”