Nitrile versus isonitrile adsorption at interstellar grain surfaces
II. Carbonaceous aromatic surfaces
1 LERMA, Sorbonne Universités, UPMC Univ. Paris 06, Observatoire de Paris, PSL Research University, CNRS, 75252 Paris, France
2 Sorbonne Universités, UPMC Univ. Paris 06, UMR – CNRS 7616, Laboratoire de Chimie Théorique, 75252 Paris Cedex 05, France
3 Institut des Sciences Chimiques de Rennes, École Nationale Supérieure de Chimie de Rennes, UMR – CNRS 6226, ENSCR, 35700 Rennes, France
Received: 9 May 2017
Accepted: 29 July 2017
Context. 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.
Aims. 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.
Methods. The question is addressed by means of a concerted experimental and theoretical approach of the adsorption energies of CH3CN and CH3NC 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 in an ultra-high vacuum between 70 and 160 K. Theoretically, the question is addressed using first-principle periodic density functional theory to represent the organised solid support.
Results. 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 (CH3CN) interacts more efficiently with the carbonaceous solid support than the higher energy isomer (CH3NC), however.
Key words: astrochemistry / ISM: molecules / ISM: abundances / methods: laboratory: molecular / methods: numerical
© ESO, 2017