Assessing realistic binding energies of some essential interstellar radicals with amorphous solid water. A fully quantum chemical approach

TL;DR

This study uses advanced quantum chemical methods to accurately estimate the binding energies of key interstellar radicals on amorphous water ice, improving astrochemical models and understanding of interstellar chemistry.

Contribution

It provides the first quantum chemical BE estimates for seven important radicals on amorphous water, revealing chemisorption and hemibonded interactions that influence their binding.

Findings

CH shows chemisorption in some sites.

Large BE for CN due to hemibonding.

Low BE for NO explains observed chemical ratios.

Abstract

In the absence of laboratory data, state-of-the-art quantum chemical approaches can provide estimates of the binding energy (BE) of interstellar species with grains. Without BE values, contemporary astrochemical models are compelled to utilize wild guesses, often delivering misleading information. Here, we employed a fully quantum chemical approach to estimate the BE of seven diatomic radicals - CH, NH, OH, SH, CN, NS, and NO - that play a crucial role in shaping the interstellar chemical composition, using a suitable amorphous solid water model as a substrate since water is the principal constituent of interstellar ice in dense and shielded regions. While the BEs are compatible with physisorption, the binding of CH in some sites shows chemisorption, in which a chemical bond to an oxygen atom of a water molecule is formed. While no structural change has been observed for the CN radical, it is believed that the formation of a hemibonded system between the outer layer of the water cluster and the radical is the reason for the unusually large BE in one of the binding sites considered in our study. A significantly lower BE for NO, consistent with recent calculations, is obtained, which helps explain the recently observed HONO/NH$_2$OH and HONO/HNO ratios in the low-mass hot corino IRAS 16293-2422 B with chemical models.