The isobaric ions CH3OPO+ and CH3OPNH2+ and their neutral counterparts: a tandem mass spectrometry and CBS-QB3 computational study

In contrast to a previous report [J. Mass Spectrom. 171 (1997) 79], the dissociative electron ionization of acephate does not generate pure m/z 78 ions CH3OPO+. By combining the results of exact mass measurements, (multiple) high energy collision experiments and CBS-QB3 calculations, it is concluded that the m/z 78 ions consist of CH3OPO+ and CH2OPOH+ in admixture with isobaric CH5PON+ ions. A computational analysis of the dissociation characteristics of the independently generated stable isomers CH3OPNH2+ and CH3P(O)NH2+ shows that the CH5PON+ ions from acephate have the structure CH3OPNH2+. As a consequence, the recovery signal in the reported neutralization–reionization (NR) spectrum of acephate’s m/z 78 ions does not prove the stability of the CH3OPO neutral. Definitive evidence for its existence as a stable species in the dilute gas phase comes from a collision-induced dissociative ionization (CIDI) experiment on the reaction C6H5P(O)OCH3+→C6H5++CH3OPO. The calculations indicate that CH3OPNH2 is also a stable species, but attempts to confirm this by experiment have not yet been successful.