N-oxides with various targets in a new hybrid mass spectrometer



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Eur. J. Mass Spectrom. 6, 3 - 9 (2000)

Collisional interaction of ionized pyridine N-oxides with various targets in a new hybrid mass spectrometer

Robert Flammang,* Virginie Henrotte and Pascal Gerbaux
Laboratory of Organic Chemistry, University of Mons-Hainaut, Avenue Maistriau 19, B-7000 Mons, Belgium

ABSTRACT:
Upon collisional activation at high kinetic energy (8 keV), the molecular ions of pyridine N-oxides 15 undergo an unexpected loss of 16 mass units (oxygen) provided molecular oxygen or nitric oxide is used as the target gas instead of helium. Molecular ions of pyridines are produced in the former experiments. This peculiar behavior seems to be correlated with the high multiplicity of the target molecules (triplet ground state for O2 and doublet for NO). Ab initio calculations suggest that a lower-lying quartet state of pyridine N-oxide ions might be involved in the oxygen-loss process. In the low-translational-energy regime (c. 2030 eV), a loss of oxygen is also detected whatever the nature of the collision gas (argon, oxygen or nitric oxide) and the relative intensity of this reaction increases with the kinetic energy of the ions. At near thermal energies (c. 5 eV), the molecular ions of the pyridine N-oxides react with nitric oxide apparently generating two different ionmolecule complexes or intermediates responsible for the production of [M + NO]+ and [M O]"+ ions. The deoxygenation mechanism appears therefore highly dependent upon the experimental conditions including target gas and kinetic energy.

Keywords: pyridine N-oxides, collisional-activation mass spectrometry, triplet oxygen, nitric oxide, molecular orbital calculations, excited states

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