The isobaric ions CH 3OPO [rad]+ and CH 3OPNH 2+ and their neutral counterparts: a tandem mass spectrometry and CBS-QB3 computational study

Abstract

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 CH 3O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 CH 3OPO + and CH 2OPOH + in admixture with isobaric CH 5PON + ions. A computational analysis of the dissociation characteristics of the independently generated stable isomers CH 3OPNH 2 + and CH 3P(O)NH 2 + shows that the CH 5PON + ions from acephate have the structure CH 3OPNH 2 +. 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 CH 3O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 C 6H 5P(O)OCH 3 +→C 6H 5 ++CH 3OPO. The calculations indicate that CH 3OPNH 2 is also a stable species, but attempts to confirm this by experiment have not yet been successful.