Pyrazine diradicals, carbenes, ylides, and distonic ions probed by theory and experiment

The [C-4,H-4,N-2] potential energy hypersurface relating to pyrazine and its hydrogen shift isomers has been investigated computationally using hybrid Hartree-Fock/density functional theory and through a variety of tandem mass spectrometry experiments (metastable ion, collision-induced dissociation, and neutralization reionization mass spectrometry). In addition to the conventional pyrazine structure 1, its alpha-ylide 2, beta-ylide 3, and the 1,4-diradical 4 were generated and characterized through neutralization reionization mass spectrometry experiments. Also, the corresponding radical cations 1(.+)- 4(.+) were accessible by dissociative electron ionization of the appropriate pyrazine esters. Quantum chemical calculations at the B3LYP/TZVP level of theory reveal that all these species correspond to minima that are separated by significant barriers thus preventing facile isomerization. As an additional, albeit high lying isomer, the 1,3-diradical 5 was computationally identified. In the case of the radical cations the energy differences between the various isomers are much smaller than for the corresponding neutrals; however, pyrazine represents in both cases the most stable species. (Int J Mass Spectrom 185/186/187 (1999) 925-933) (C) 1999 Elsevier Science B.V.