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Pyrazole N ‐methylide radical cation: ion–molecule reactions in a new hybrid tandem mass spectrometer and DFT molecular orbital calculations
Author(s) -
Flammang Robert,
BarbieuxFlammang Monique,
Haverbeke Yves Van,
Luna Alberto,
Tortajada Jeanine
Publication year - 2000
Publication title -
journal of physical organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.325
H-Index - 66
eISSN - 1099-1395
pISSN - 0894-3230
DOI - 10.1002/(sici)1099-1395(200001)13:1<13::aid-poc196>3.0.co;2-d
Subject(s) - chemistry , moiety , molecule , ion , reactivity (psychology) , mass spectrometry , density functional theory , methylene , photochemistry , pyridine , mass spectrum , molecular orbital , acetonitrile , computational chemistry , medicinal chemistry , stereochemistry , organic chemistry , medicine , alternative medicine , pathology , chromatography
The gas‐phase reactivity of the classical ionized N ‐ or C ‐methylpyrazoles and the corresponding distonic pyrazolium N ‐methylide ions a and distonic‐like pyrazolium C ‐methylide ions b , generated from several pyrazole derivatives, 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 higher stability of the isomers a and b ions compared with the classical ionized N ‐ or C ‐methylpyrazoles. In the light of the theoretical results, the loss of a neutral methylene group from ions a and b was predicted to be energetically disfavoured, in agreement with experimental findings. Ion–molecule reactions performed in a new hybrid mass spectrometer of sectors–quadrupole–sectors (EBEqEBE) configuration, were used for the characterization of the [C 4 H 6 N 2 ] ·+ ions studied. The targets used were nitric oxide, dimethyl disulphide, dimethyl diselenide, pyridine and acetonitrile. In the specific case of ions a , generated from N ‐acetylpyrazole, the presence in the mass spectra (or in the collisional activation mass spectra of mass‐selected ion–molecule reaction products) of CH 2 NO + , CH 3 SCH 2 + or CH 3 CNCH 2 ·+ fragment ions confirms the presence of the CH 2 moiety and therefore their distonic character. The most stable ions b proposed to be produced from 3(5)‐ N,N ‐dimethylaminopyrazole do not show the same reactivity (i.e. the CH 2 moiety transfer). Because all the [C 4 H 6 N 2 ] ·+ 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 and the ionization energies of neutral methylpyrazoles were also calculated. Copyright © 2000 John Wiley & Sons, Ltd.