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

Abstract

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.

Probing Coherent Vibrations of Organic Phosphonate Radical Cations with Femtosecond Time-Resolved Mass Spectrometry

“…The dissociation products analyzed in Figure 3 have been observed in numerous mass spectrometry studies of DMMP [23,24,25,26,27,28,29,30,31]. Their assigned structures based on these studies and our recently reported theoretical calculations [31] are given in Scheme 2.…”

“…The associated vertical ionization potentials (IPvert) and cation relaxation energies (Erelax) for each molecule are listed in Table 1. We note that the location of the unpaired electron in the relaxed cations remained on the oxygen atom initially double-bonded to the phosphorus atom; the known radical migrations in these molecules are associated with hydrogen atom transfers shown in Scheme 3 and Scheme 5 [24,25,26,27,28,29,30,31,32].…”

A Theoretical and Mass Spectrometry Study of Dimethyl Methylphosphonate: New Isomers and Cation Decay Channels in an Intense Femtosecond Laser Field

“…33−37 The DMMP radical cation (1) has been found to spontaneously undergo a series of 1,4-hydrogen atom shifts that results in isomerization to its enol form (3) (see Scheme 1), which is observationally stable over tens of microseconds. 36,37 The intermediate 2 can dissociate into PO 2 C 2 H 7 + (m/z = 94) via low-energy loss of CH 2 O, and similar dissociation pathways have been observed in related compounds. 2,35,36,38 While the complex reactivity of the DMMP radical cation has been known for decades, a thorough computational ab initio investigation of the cation decay pathways involved has not yet been performed.…”

An ab initio and DFT study of the fragmentation and isomerisation of MeP(O)(OMe)+

“…The structure of one of the postulated intermediate ions, MeOPOMe þ has been studied previously at the B3LYP level with a smaller basis set than that used in the present work. 3 Mass spectrometric 4,5 and computational 5 studies of phosphonate ions have been reported but as these were concerned with odd-electron ions it is difficult to relate the results of these studies to the present work although superficially they appear similar.…”