FRAGMENTATION OF PROTONATED O,O-DIETHYL O-ARYL PHOSPHOROTHIONATES IN TANDEM MASS-SPECTRAL ANALYSIS

The gas-phase ion chemistry of protonated O,O-diethyl O-aryl phosphoro thionates was studied with tandem mass spectrometric and ab initio the oretical methods. Collision-activated dissociation (CAD) experiments w ere performed for the [M + H](+) ions on a triple quadrupole mass spec trometer. Various amounts of internal energy were deposited into the i ons upon CAD by variation of the collision energy and collision gas pr essure. In addition to isobutane, deuterated isobutane C4D10 also was used as reagent gas in chemical ionization. The daughter ions [M (+) H - C2H4](+) and [M + H - 2C(2)H(4)](+) dominate the CAD spectra. These fragments arise via various pathways, each of which involves gamma-pr oton migration. Formation of the terminal ions [M + H - 2C(2)H(4) - H2 O](+), [M + H - 2C(2)H(4) - H2S](+), [ZPhOH(2)](+), [ZPhSH(2)](+), and [ZPhS](+) [Z = substituent(s) on the benzene ring] suggests that (1) the fragmenting [M + H](+) ions of O,O-diethyl O-aryl phosphorothionat es have protons attached on the oxygen of an ethoxy group and on the o xygen of the phenoxy group; (2) thiono-thiolo rearrangement by aryl mi gration to sulfur occurs; (3) the fragmenting rearranged [M + H](+) io ns have protons attached on the oxygen of an ethoxy group and on the s ulfur of the thiophenoxy group. To get additional support for our inte rpretation of the mass spectrometric results, some characteristics of three protomers of O,O-diethyl O-phenyl phosphorothionate were investi gated by carrying out ab initio molecular orbital calculations at the RHF/3-21G level of theory.