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 CD,-labeled D M M P 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 I ,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 D M M P molecular ion. The thermodynamic data show that the energy barrier to isomerization is below the internal energy required for decomposition of the D M M P molecular ion. Additionally, the AH," for the intermediate and enolic isomers are shown to be significantly less than the AH," for the keto form of the D M M P molecular ion. CH3-C -OCHZ* INTERMEDIATE CH3-P-OCH2.
Figure 2. Isomers of the methyl acetate molecular ion and the analogous isomers for the molecular ion of DMMP.