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 it s hydrogen shift isomers has been investigated computationally using hybrid Hartree-Fock/density functional theory and through a variety of tandem mas s spectrometry experiments (metastable ion, collision-induced dissociation, and neutralization reionization mass spectrometry). In addition to the con ventional pyrazine structure 1, its alpha-ylide 2, beta-ylide 3, and the 1, 4-diradical 4 were generated and characterized through neutralization reion ization mass spectrometry experiments. Also, the corresponding radical cati ons 1(.+)- 4(.+) were accessible by dissociative electron ionization of the appropriate pyrazine esters. Quantum chemical calculations at the B3LYP/TZ VP 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 comput ationally identified. In the case of the radical cations the energy differe nces between the various isomers are much smaller than for the correspondin g neutrals; however, pyrazine represents in both cases the most stable spec ies. (Int J Mass Spectrom 185/186/187 (1999) 925-933) (C) 1999 Elsevier Sci ence B.V.