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 i ons of pyridine N-oxides 1-5 undergo an unexpected loss of 16 mass units (o xygen) provided molecular oxygen or nitric oxide is used as the target gas instead of helium. Molecular ions of pyridines are produced in the former e xperiments. This peculiar behavior seems to be correlated with the high mul tiplicity of the target molecules (triplet ground state for O-2 and doublet for NO). Ab initio calculations suggest that a lower-lying quartet state o f pyridine N-oxide ions might be involved in the oxygen-loss process. In th e low-translational-energy regime (c. 20-30 eV), a loss of oxygen is also d etected whatever the nature of the collision gas (argon, oxygen or nitric o xide) and the relative intensity of this reaction increases with the kineti c energy of the ions. At near thermal energies (c. 5 eV), the molecular ion s of the pyridine N-oxides react with nitric oxide apparently generating tw o different ion-molecule complexes or intermediates responsible for the pro duction of [M + NO](+) and [M - O](.+) ions. The deoxygenation mechanism ap pears therefore highly dependent upon the experimental conditions including target gas and kinetic energy.