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 rearra nges to its more stable alpha-ylide isomer by an ion-molecule reaction with a suitable reagent, such as 2-cyanopyridine. The initially formed [pyridin e(.+) ...2-cyanopyridine] adduct isomerizes, by way of proton-transport cat alysis, to a stable complex of 2-cyanopyridine with the cc-ylide ion which may dissociate. Multiple collision experiments on deuterium labelled pyridi nes indicate that a further isomerization may occur: about half of the (met astable) complex ions undergo a cyano-transfer leading to a very stable dis tonic ion. A subsequent 1,4-hydrogen shift may lead to the di-2-pyridyl ket imine ion which could account for the observed survivor signal in the neutr alization-reionization mass spectrum of the complex. Ab initio calculations at the RHF/6-31G(d) level of theory fully support these findings and provi de a rationale for the observed cyanotransfer reaction.