Abstract
Extensive ^13^C and ^15^N labelling has shown that the molecular ions of 2‐, 3‐ and 4‐cyanopyridine with lifetimes up to 10^−6^ s eliminate hydrogen cyanide originating predominantly from the ring (˜65%). Moreover, this hydrogen cyanide loss occurs after an equilibrated positional interchange of the ring carbon atoms at positions interchange of the ring carbon atoms at positions 2, 4 and 6 via Dewar pyridine structures. In molecular ions with lifetimes of 10^−6^–10^−5^ s skeletal rearrangements have taken place in such a way that both nitrogen atoms have become equivalent prior to the loss of hydrogen cyanide. Arguments are put forward that this equivalence of nitrogen atoms is caused by the intermediacy of ions with a 1,4‐dicyanobuta‐1,3‐diene structure. About 60% of these intermediate ions eliminate hydrogen cyanide in a fast process. The remaining 40% of these ions undergo ring closure again to a pyridine ring in which the carbon atoms of positions 2, 4 and 6 are positionally interchanged rapidly via Dewar pyridine structures followed by ring opening again and eventual loss of hydrogen cyanide. This interpretation of the ^13^C and ^15^N labelling results is further corroborated by a study of the loss of hydrogen cyanide from molecular ions of 1,4‐dicyanobuta‐1,3‐diene labelled with ^13^C in both cyano groups.