The covalently bound HCN dimer ions HCN−NCH+ and HCN−C(N)H+ are stable species in the gas-phase, but the neutral counterparts are not

Tandem mass spectrometry based experiments and computational chemistry (CBS-QB3/APNO methods) indicate that ionized s-tetrazine eliminates N2 to yield the elusive linear HCN dimer ion HCN–NCH+ (1). The ion is stable in the rarefied gas-phase but, as it is generated with considerable internal energy, it readily interconverts with HCN–C(N)H+ (2), via a 1,2-HCN shift. This more stable HCN dimer ion was generated independently by loss of HCN from ionized s-triazine. Whereas the HNC dimer ion HNCCNH+ has a (kinetically) stable neutral counterpart, theory and experiment (neutralization–reionization mass spectrometry) agree that the HCN dimer ions 1 and 2 do not.