Binding Energies of N-Bearing Astrochemically-Relevant Molecules on Water Interstellar Ice Models. A Computational Study

Abstract

Different molecules and radical species have been detected and identified in the interstellar medium (ISM) through rotational and infrared techniques. The presence of dust grains in ISM have also been observed, which in the denser regions are covered by thick ice mantles consisting mainly of water molecules. Quantifying the interaction of gas phase species with water ice is essential to accurately model the evolution of the ISM. Hence, the importance to obtain accurate binding energies. In this contribution, the binding energies (BEs) of different N-containing interstellar species on water ice surfaces are presented. They have been calculated by means of reliable periodic quantum chemical simulations. Two different methods have been used: i) hybrid DFT methods, i.e., B3LYP for closed-shells species and M06-2X for open-shell radicals, and ii) the cost-effective DFT//HF-3C, i.e., geometry optimization at HF-3c followed by a single point energy refinement at the DFT level. As a first step, BEs have been computed on a crystalline water ice surface model of a thickness of 12 atomic layers, in which different super cells have been adopted (involving 36 or 72 water molecules per unit cell), depending on the size of the adsorbate. Moreover, an ONIOM-like local approximation combining CCSD(T) with DFT functional as high and low theory levels, respectively, has been adopted to correct BEs.