Recently, ultrafast pump-probe mass spectroscopy was used to study the vibrational dynamics of the ionized nerve agent stimulant dimethyl methylphosphonate (DMMP). Upon ionization, the parent radical cation (DMMP+) exhibits ~45 fs oscillations in its transient ion yield with varying pump-probe delay. An antiphase relation was observed in between the dynamics of the DMMP+ and other fragment cations. To understand the experimental observation, we performed high level theoretical calculations. Electronic structure calculations of the neutral and the cation reveal there are three stable conformers that can be involved in the dynamics, although they all exhibit similar behavior. Ionization potentials of DMMP and the oscillator strengths for the transitions from ground state to excited states of the cation were calculated at the coupled cluster level along the linearly interpolated geometries between the neutral and cationic optimized geometries. It was found that as the cation oscillates the probe light becomes resonant with the 2nd or 3rd excited state of the cation, driving excitation to these states. These results suggest that the probe light excites the DMMP+ from its ground state to one or both of these higher excited states, causing depletion of the parent ion and the formation of fragment ions which will oscillate with a π phase shift.

Theoretical study of dimethyl methyl phosphonate (DMMP): Interpreting ultrafast pump-probe experiments

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

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