Tandem mass spectrometry based experiments and computational chemistry (CBS-QB3/APNO methods) indicate that ionized s-tetrazine eliminates N-2 to yield the elusive linear HC N dimer ion HC=N-N=CH center dot+ (1). The ion is stable in the rarefied gas-phase but, as it is generated with considerable internal energy, it readily interconverts with HC=N-C(=N)H center dot+ (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 HN C dimer ion HN=C=C=NH center dot+ has a (kinetically) stable neutral counterpart, theory and experiment (neutralization-reionization mass spectrometry) agree that the HC N dimer ions 1 and 2 do not. (c) 2009 Elsevier B.V. All rights reserved.