Pyrimidine-ylidenes produced using neutralization-reionization mass spectrometry and probed by density functional methods

The potential energy surface comprising ionized pyrimidine, 1(.+), and eigh t of its hydrogen-shift isomers, as well as that of the corresponding neutr als was explored at a level of theory (B3LYP/TZVP) that has proven adequate for related species. The computations predicted that among the isomers the re are four C4H4N2.+ distonic radical cations, 2(.+)-5(.+), of comparable 2 stability to 1(.+), and transition state calculations indicated that high barriers separate these stable ions. Thus, the ions 2(.+)-5(.+) should also be viable chemical species, and indeed mass spectrometry based experiments lead to the generation and characterization of three of the four, that is 2(.+), 3(.+), and 4(.+), as stable ions in the gas phase. Ions 2(.+)-4(.+) were identified on the basis of their collision-induced dissociation charac teristics in the mass spectrometer. The ions 2(.+) and 3(.+) obtained by di ssociative electron impact ionization were subjected to neutralization-reio nization mass spectrometry (NRMS). From collision-induced dissociation spec tra of the intense NRMS survivor ions, it follows that the neutral ylide/ca rbene counterparts, i.e. pyriniidine-4-ylidene, 2, and pyrintidine-2-yliden e, 3, have lifetimes of at least microseconds in the rarefied gas phase. Th e interpretation of the experimental observations that 2 and 3 are viable c hemical species in gaseous environs was supported computationally. Accordin g to the calculations the neutral isomers 2-5 each represent a minimum sepa rated by high hydrogen-shift barriers, although situated some 50 kcal/mol h igher in energy than 1, pyrimidine itself. However, molecules 4 and 5 remai ned elusive since ions 4(.+), only obtainable by a collision-induced dissoc iation process, were not amenable to NR experiments and a viable strategy t o produce a beam of pure ions 5(.+) could not be realized. (C) 2001 Elsevie r Science B.V.