  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.
Among these 37 molecules, 30 are nitrile R-CN compounds, the remaining seven
belonging to the isonitrile R-NC family. How these species behave in presence
of the grain surfaces is still an open question. In this contribution we
investigate 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 different nature and morphologies.
The question was addressed by means of a concerted experimental and theoretical
study of the adsorption energies of CH3CN and CH3NC on the surface water ice
and silica. The experimental determination of the molecule - surface
interaction energies was carried out using temperature programmed desorption
(TPD) under an ultra-high vacuum (UHV) between 70 and 160 K. Theoretically, the
question was addressed using first principle periodic density functional theory
(DFT) to represent the organized solid support. The most stable isomer (CH3CN)
interacts more efficiently with the solid support than the higher energy isomer
(CH3NC) for water ice and silica. Comparing with the HCN and HNC pair of
isomers, the simulations show an opposite behaviour, in which isonitrile HNC
are more strongly adsorbed than nitrile HCN provided that hydrogen bonds are
compatible with the nature of the model surface. The present study confirms
that the strength of the molecule surface interaction between isomers is not
related to their intrinsic stability but instead to their respective ability to
generate different types of hydrogen bonds.
