Fragmentations and reactions of protonated O,O-dimethyl ethylphosphonate and some isotopomers produced by electrospray ionisation in an ion trap mass spectrometer

The fragmentation behaviour of protonated O,O-dimethyl ethylphosphonate and its isotopomers deuterated in the α- and β-positions of the ethyl group and their fragment ions, particulary EtP(O)OMe+(IV), have been investigated both experimentally in an ion trap mass spectrometer and theoretically by electronic structure calculations at the B3LYP level. Of particular interest is the finding that the phosphonium ion IV eliminates ethene with hydrogen/deuterium loss from both the α-and β-positions. The initial step for both routes involves ethyl migration from P to O to form the ion MeOP+OEt which then loses ethene by two mechanisms, both of which lead to the same products. That a unitary branching ratio for α- and β-elimination is not observed indicates that although the final step of dissociation into product ion and ethene is energetically the most demanding, it is not rate limiting and the large entropy change associated with the dissociation allows earlier processes to determine the branching ratio. This demonstrates once again that free energy, not enthalpy (or energy), determines the course of gas phase ion processes.