Structures and dissociation mechanisms of protonated and electron ionized methamidophos

Ahmad Rifai (1, 2) , Sophie Bourcier (1) , Farouk Jaber (2) , Guy Bouchoux (1)

1 DCMR - Laboratoire des mécanismes réactionnels
2 Laboratoire d'Analyse de Pesticides et de Polluants Organiques

Methamidophos, 1, is examined by tandem mass spectrometry (Q-TOF) under electrospray ionization in the positive ion mode (ESI-P). Variable collision energy MS-MS mass spectra of protonated methamidophos, 1H(+), reveal that CH2O, CH3SH, CH3OH and NH3 losses occur with comparable threshold energies. Fragmentations leading to m/z 125, 112,110, 94, 79 and m/z 64 ions are explained by combinations of 1,2-elimination reactions and isomerizations via 1,3-proton migrations as supported by B3LYP/6-31+G(d) calculations. The behavior of electron ionized methamidophos, 1(center dot+), particularly its fragmentations by losses of (SCH3)-S-center dot and CH2S, is also explored at the B3LYP/6-31+G(d) level. G4MP2 calculation shows that protonation of 1 occurs preferentially at the oxygen atom of the P=O group with a corresponding proton affinity equal to PA(1) = 897 kJ mol(-1). Calculated G4MP2 ionization energy and heat of formation of methamidophos are IE(1) = 8.77 eV and Delta H-f(298)degrees(1)=-626 kJ mol(-1). (Cop) 2013 Elsevier B.V. All rights reserved.

Ahmad Rifai, Sophie Bourcier, Farouk Jaber, Guy Bouchoux. Structures and dissociation mechanisms of protonated and electron ionized methamidophos. International Journal of Mass Spectrometry, 2013, 339-340, pp.7-15. ⟨10.1016/j.ijms.2013.02.003⟩. ⟨hal-00880175⟩