Does the ion–molecule reaction between HCCH+ and HCN lead to CH2CH–CN+? A computational and experimental study of the reverse process

The title ion–molecule reaction has been proposed to play an important role in interstellar chemistry if it yields acrylonitrile ions CH2CH–CN+. This question was probed by examining the formation of HCCH+ and HCN from low-energy ions CH2CH–CN+ and related isomers, using tandem mass spectrometry based experiments (D and 13C labelling) in conjunction with model chemistry calculations (CBS-QB3/APNO). We conclude that the title reaction is a barrierless multistep rearrangement that may not effectively compete with the straightforward formation of stable distonic ions HCCH–NCH+ from HCCH+(ion)–HCN(dipole) encounter complexes.