Binding Energies of N-bearing Species on Interstellar Water Ice Mantles by Quantum Chemical Calculations

Abstract

Of the about 300 gas-phase molecular species so far detected in the interstellar medium (ISM), mostly via observations of their rotational lines, around 40% contain nitrogen (N) atoms. Likewise, of the less than a dozen interstellar molecules, firmly or likely detected in the solid-state water-dominated icy matrix by means of infrared observations, two bear N. A crucial parameter that regulates whether a species is in the gas or adsorbed on the icy phase is their binding energy (BE) toward the icy grain. Therefore, an accurate quantification of the BE is of paramount importance to properly model the ISM chemistry through numerical models. However, very few BEs are available in the literature, either determined experimentally or theoretically. In the present study, we calculate the BEs of 21 among the most abundant interstellar N-bearing species. We adopted two structural water ice models, representing a crystalline and an amorphous surface, using a reliable cost-effective procedure based on the density functional theory. While on the crystalline surface model only one BE per species is obtained due to the high symmetry of the unit cell, on the amorphous model from 5 to 10 BEs are obtained, due to its richer surface morphological variety. Most of our computed BEs agree with available experimental and other computational values. Finally, we discuss how the newly computed BEs can help estimate which N-bearing species can be frozen at the water snow line and, therefore, incorporated in water-rich ice planetesimals.

The Complex (Organic) Puzzle of the Formation of Hydrogen Cyanide and Isocyanide on Interstellar Ice Analogues

“…The distribution is bimodal, representing both hydrogen bonding and hemibonding. With 85% the latter dominates the distribution with a mean binding energy value of 48.6 kJ mol –1 , extending from 27.0 to 74.7 kJ mol –1 , closely aligning with recent results by Martinez-Bachs et al, which report a binding energy range of 21.7 to 64.8 kJ mol –1 , and the work by Rimola et al, who observed a binding energy of 76.6 kJ mol –1 in a cavity-like structure of a 33-water-molecule cluster, characterized by a high number of intermolecular interactions. This value is compatible with the high end of our distribution and the difference is likely to be produced by the different theory level, where the density functional BHandHLYP was used in combination with a double-ζ basis set for geometries and zero point energies and a triple-ζ one for energy refinement.…”