Nitrile versus isonitrile adsorption at interstellar grain surfaces II. Carbonaceous aromatic surfaces

Almost 20 % of the ~ 200 different species detected in the interstellar and circumstellar media present a carbon atom linked to nitrogen by a triple bond. Of these 37 molecules, 30 are nitrile R-CN compounds, the remaining 7 belonging to the isonitrile R-NC family. How these species behave in their interactions with the grain surfaces is still an open question. In a previous work, we have investigated whether the difference between nitrile and isonitrile functional groups may induce differences in the adsorption energies of the related isomers at the surfaces of interstellar grains of various nature and morphologies. This study is a follow up of this work, where we focus on the adsorption on carbonaceous aromatic surfaces. The question is addressed by means of a concerted experimental and theoretical approach of the adsorption energies of CH$_3$CN and CH$_3$NC on the surface of graphite (with and without surface defects). The experimental determination of the molecule and surface interaction energies is carried out using temperature-programmed desorption (TPD) in an ultra-high vacuum (UHV) between 70 and 160 K. Theoretically, the question is addressed using first-principle periodic density functional theory (DFT) to represent the organised solid support. The adsorption energy of each compound is found to be very sensitive to the structural defects of the aromatic carbonaceous surface: these defects, expected to be present in a large numbers and great diversity on a realistic surface, significantly increase the average adsorption energies to more than 50\% as compared to adsorption on perfect graphene planes. The most stable isomer (CH$_3$CN) interacts more efficiently with the carbonaceous solid support than the higher energy isomer (CH$_3$NC), however.