Generation and Characterization of Ionic and Neutral [HS-P-OH]+/· and S=P(OH)2+/· in the Gas Phase by Tandem Mass Spectrometry and Computational Chemistry

Dissociative electron ionization of diethyl dithiophosphate (I) and O,O′-diethyl methylphosphonothioate (II) generates moderately abundant m/z 81 ions of composition [P, O, S, H2]+. From tandem mass spectrometry experiments and theoretical calculations at the B3LYP/6-31G(d,p), G2, and G2 (MP2) levels it is concluded that the majority of the ions have the structure of HS-P-OH+ (1a+) and it is separated by high-energy barriers from its isomers P(= S)OH2+ (1b+), P(= O)SH2+ (1c+), HP(= S)OH+ (1d+), and HP(= O)SH+ (1e+). Low-energy (metastable) ions 1a+ dissociate via losses of H2O and H2S to yield m/z 63 (PS+) and m/z 47 (PO+) product ions, respectively. These reactions involve isomerization of 1a+ into the stable isomers 1b+ and 1c+. Neutralization-reionization experiments confirm the theoretical prediction that radical 1a· is a stable species in the gas-phase. Variable-time NR experiments indicated that only a small fraction of metastable 1a· radicals dissociate in the 0.4-4.6 μs time window, while most dissociations occurred on a shorter time scale. RRKM calculations were performed to investigate unimolecular dissociation kinetics of 1a· which were found to be in agreement with the fragmentation observed in the NR spectrum. The 70-eV electron ionization of (I) and diethyl chlorothiophosphate (III) yields m/z 97 ions, predominantly of the structure S = P(OH)2+ (2a+). This conclusion follows from tandem mass spectrometry experiments and theoretical calculations. The calculations predict that (2a+) is separated by high-energy barriers from its isomers O = P(SH)OH+ (2b+), S = P(= O)OH2+ (2c+), and O = P(= O)SH2+ (2d+). Neutralization-reionization experiments confirmed that 2a· radical is a kinetically stable species on the time scale of up to 5 μs, which is in agreement with ab initio calculations. However, owing to a mismatch of Franck-Condon factors a large fraction of 2a· dissociates by loss of SH· yielding O = P − OH.