Structures and dissociation mechanisms of protonated and electron ionized methamidophos

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

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