  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.
