  The cyano radical (CN) is one of the most frequently remotely observed
species in space, also in comets. Data from the high-resolution Double Focusing
Mass Spectrometer (DFMS) on board the Rosetta orbiter, collected in the inner
coma of comet 67P/Churyumov-Gerasimenko, revealed an unexpected chemical
complexity, and, recently, also more CN than expected from photodissociation of
its most likely parent hydrogen cyanide (HCN). This work is dedicated to the
derivation of abundances relative to HCN of three cometary nitriles (including
structural isomers) from DFMS data. Mass spectrometry of complex mixtures does
not always allow distinction of structural isomers. We assumed the most stable
and most abundant (in similar environments) structure in our analysis, that is
HCN for CHN, CH3CN for C2H3N, HC3N for C3HN, and NCCN for C2N2. For
cyanoacetylene (HC3N) and acetonitrile (CH3CN) the complete mission timeline
was evaluated, while cyanogen (NCCN) often was below detection limit. By
carefully selecting periods where cyanogen was above detection limit, we were
able to follow the abundance ratio between NCCN and HCN from 3.16 au inbound to
3.42 au outbound. These are the first measurements of NCCN in a comet.We find
that neither NCCN, nor any of the other two nitriles, is sufficiently abundant
to be a relevant alternative parent to CN.
