Collisional Interaction of Ionized Pyridine N -Oxides with Various Targets in a New Hybrid Mass Spectrometer

Upon collisional activation at high kinetic energy (8 keV), the molecular ions of pyridine N-oxides 1–5 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. 20–30 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 ion–molecule 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.