CCSD(T)-F12a Study of Reactions of Interstellar Anions C2nH-(n=2-4) with HCCH

Explicitly correlated coupled cluster theory at the CCSD(T)-F12a level (Adler, T. B.; Knizia, G.; Werner, H.-J. J. Chem. Phys. 2007, 127, 221106) was employed to study the energetics of reactions of interstellar anions C2nH- with HCCH, which may formally be regarded as proton-transfer reactions. The gas-phase acidities at 0 K for the polyynes HC2nH with n = 2-4 are predicted to be 1516.5, 1483.2, and 1462.6 kJ mol(-1). The energy profiles of the collinear reactions are characterized by absolute minima corresponding to the hydrogen-bonded species HC2n-center dot center dot center dot HCCH, saddle points at which the proton is shared between the two anion moieties, and a second minimum in the exit channel of the reaction, which becomes less pronounced with increasing n.