Lowering Large 1,2-H Shift Barriers by Proton-Transport Catalysis: The Challenging Case of the Pyridine Radical Cation

Under conditions of chemical ionization, the pyridine radical cation rearranges to its more stable α-ylide isomer by an ion–molecule reaction with a suitable reagent, such as 2-cyanopyridine. The initially formed [pyridine•+•••2-cyanopyridine] adduct isomerizes, by way of proton-transport catalysis, to a stable complex of 2-cyanopyridine with the α-ylide ion which may dissociate. Multiple collision experiments on deuterium labelled pyridines indicate that a further isomerization may occur: about half of the (metastable) complex ions undergo a cyano-transfer leading to a very stable distonic ion. A subsequent 1,4-hydrogen shift may lead to the di-2-pyridyl ketimine ion which could account for the observed survivor signal in the neutralization–reionization mass spectrum of the complex. Ab initio calculations at the RHF/6-31G(d) level of theory fully support these findings and provide a rationale for the observed cyano-transfer reaction.