Keto-to-enol isomerization in the molecular ion of dimethylmethylphosphonate

A mechanism for keto-to-enol isomerization in dimethylmethylphosphonate (DMMP) has been proposed based on deuterium-labeling studies, a model compound and thermodynamic data. An electron ionization study of H/D exchange occurring in CD₃-labeled DMMP suggests that rapid keto-to-enol isomerization occurs in the ion source and a reaction mechanism based on sequential 1,4-H migrations rather than by the direct 1,3-H transfer or by sequential 1,2-H migrations is proposed. The examination of the mass-analyzed ion kinetic energy/collision-induced dissociation spectrum of the methylphosphonic acid molecular ion suggests that keto-to-enol isomerization does not occur for this species and that 1,2- and 1,3-H migrations are not favored. Available thermodynamic data were employed to construct a potential energy surface for keto-to-enol isomerization of the DMMP molecular ion. The thermodynamic data show that the energy barrier to isomerization is below the internal energy required for decomposition of the DMMP molecular ion. Additionally, the ΔH_f° for the intermediate and enolic isbmers are shown to be significantly less than the ΔH_f° for the keto form of the DMMP molecular ion.