  Experimental and theoretical studies are presented on the reactivity of the
radical cation isomers methanimine and aminomethylene with ethyne. Selective
isomer generation is performed via dissociative photoionization of suitable
neutral precursors and via direct photoionization of methanimine. Reactive
cross sections and product branching ratios are measured as a function of
photon and collision energies. Results are discussed in light of ab initio
calculations of reaction mechanisms. The major channels, for both isomers, are
due to H atom elimination from covalently bound adducts to give [C3NH4]+.
Theoretical calculations show that while for the reaction of aminomethylene
with acetylene any of the three lowest energy [C3NH4]+ isomers can form via
barrierless and exothermic pathways, for the methanimine reagent the only
barrierless pathway is the one leading to the production of protonated vinyl
cyanide (CH2CHCNH+), a prototypical branched nitrile species that has been
proposed as a likely intermediate in star forming regions and in the atmosphere
of Titan. The astrochemical implications of the results are briefly addressed.
