A Computational Investigation into the Hydrogenation of NO on Water Ice Surfaces to Rationalize the NO:HNO:NOH Disparity in Space

Abstract

Molecules possessing NO bonds are important precursors in biology; however, in the interstellar medium (ISM), such bonds are observed in relatively low abundance. NO and HNO are two species detected concurrently within the ISM that contain this bond in its simplest form. However, NO is observed to be approximately 200 times more abundant than HNO, which is curious due to the ubiquity of hydrogenated species in the ISM. In the present work, we use computational techniques to determine whether (i) HNO can be formed from NO on cold dust grain surfaces and (ii) if formed, HNO is able to desorb from the surface. Dust grains, which at low (10 K) temperature are primarily coated in water ice, are modeled using both hexagonal and amorphous ice models. A strong thermodynamic driving force is calculated for NO hydrogenation to HNO on the water ice surfaces, suggesting facile formation of HNO. Interestingly, the formation of NOH is also a thermodynamic possibility. Investigation into the reaction kinetics showed no barrier to hydrogenation in either scenario on the hexagonal ice surface; however, on the more astronomically relevant amorphous ice, barriers of 0.53 eV (51 kJ mol–1) and 0.77 eV (74 kJ mol–1) are observed for the formation of HNO and NOH, respectively. Comparison of the adsorption energies showed NO to bind to the surface the weakest (< –0.2 eV/–19 kJ mol–1), followed by HNO (< –0.6 eV/–58 kJ mol–1), and then NOH (< –1.50 eV/–144 kJ mol–1). This suggests that, once formed, HNO is likely to remain adsorbed to the surface, thus accounting for the lower gas phase abundance observed compared to NO. NOH, if formed, will be even less likely to desorb in appreciable amounts and hence remains undetected. Combined, the results suggest that hydrogenated species are possible to be formed from NO on ice surfaces and are likely to remain bound to the surface, supporting the experimental observations.