The CBS-QB3 model chemistry predicts that the title ion-molecule reaction, of potential interest in astrochemistry, yields a stable head-to-tail dimer, [HC=N-CH2C(H)C N](center dot+) (D1). Cyclization of D1 into ionized pyrimidine seems possible, but the initiating 1,2-H shift is close in energy to back-dissociation into CH2=C(H)CN center dot+ (AN) + HCN. Less energy demanding is formation of the H-bridged isomers [CH2=C(CN)H- -N CH](center dot+) and [HC N- -HC(H)=C(H)CN](center dot+), whose HCN component may catalyze isomerization of AN into CH2=C=C=NH center dot+ (AN1) and CH=C(H)C=NH center dot+ (AN2) respectively. Tandem mass spectrometry based experiments using N-15/C-13 labelling show that cyclization of D1 does not occur and that AN1 is the predominant reaction product instead. (C) 2009 Elsevier B. V. All rights reserved.