Abstract
Moleculespossessing NO bonds are important precursors in biology;however, in the interstellar medium (ISM), such bonds are observedin relatively low abundance. NO and HNO are two species detected concurrentlywithin 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 theISM. In the present work, we use computational techniques to determinewhether (i) HNO can be formed from NO on cold dust grain surfacesand (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 strongthermodynamic driving force is calculated for NO hydrogenation toHNO 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 hydrogenationin either scenario on the hexagonal ice surface; however, on the moreastronomically relevant amorphous ice, barriers of 0.53 eV (51 kJmol(-1)) and 0.77 eV (74 kJ mol(-1)) are observed for the formation of HNO and NOH, respectively. Comparisonof 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/-144kJ mol(-1)). This suggests that, once formed, HNOis likely to remain adsorbed to the surface, thus accounting for thelower gas phase abundance observed compared to NO. NOH, if formed,will be even less likely to desorb in appreciable amounts and henceremains undetected. Combined, the results suggest that hydrogenatedspecies are possible to be formed from NO on ice surfaces and arelikely to remain bound to the surface, supporting the experimentalobservations.