Pyrazole N-methylide radical cation: ion-molecule reactions in a new hybrid tandem mass spectrometer and DFT molecular orbital calculations

The gas-phase reactivity of the classical ionized N- or C-methylpyrazoles a nd the corresponding distonic pyrazolium N-methylide ions a and distonic-li ke pyrazolium C-methylide ions b, generated from several pyrazole derivativ es, was investigated through the use of mass spectrometric techniques and a density functional theory (B3LYP DFT) approach. The calculations performed at the B3LYP/6-311 + G(3df, 2p)//B3LYP/6-31G(d,p) + ZPE level showed the h igher stability of the isomers a and b ions compared with the classical ion ized Nor C-methylpyrazoles. In the light of the theoretical results, the lo ss of a neutral methylene group from ions a and b was predicted to be energ etically disfavoured, in agreement with experimental findings. Ion-molecule reactions performed in a new hybrid mass spectrometer of sectors-quadrupol e-sectors (EBEqEBE) configuration, were used for the characterization of th e [C4H6N2](.+) ions studied. The targets used were nitric oxide, dimethyl d isulphide, dimethyl diselenide, pyridine and acetonitrile. In the specific case of ions a, generated from N-acetylpyrazole, the presence in the mass s pectra (or in the collisional activation mass spectra of mass-selected ion- molecule reaction products) of CH2NO+, CH3SCH2+ or CH3CNCH2.+ fragment ions confirms the presence of the CHP moiety and therefore their distonic chara cter. The most stable ions b proposed to be produced from 3(5)-N,N-dimethyl aminopyrazole do not show the same reactivity (i.e. the CH2 moiety transfer ). Because all the [C4H6N2](.+) ions are able to protonate pyridine and not acetonitrile, and because these ions do not react with pyridine by charge exchange, the deprotonation energies of the ionized and distonic species an d the ionization energies of neutral methylpyrazoles were also calculated. Copyright (C) 2000 John Wiley & Sons, Ltd.