Infrared and laser-Raman spectra of 1,2-difluoroethane (FH2CCH2F) have been recorded from 4000 to 50 cm−1. Whereas the Raman spectra were measured for all three physical states, the infrared data were recorded for the gaseous and crystalline phases. Spectral transitions associated with the predominant gauche conformer have been assigned with confidence on the basis of clearly defined infrared band contours and Raman depolarization ratios. Although several fundamentals have also been assigned for trans-1,2-difluoroethane, the majority of the normal modes associated with trans-C2F2H4 appear to be unresolved or coincident with bands assigned to gauche-C2F2H4. The magnitude of ΔH separating the two conformers was determined to be 1.98±0.08 kcal/mole by measuring the temperature dependence of the Raman bands assigned to the F–C–C–F bending modes for gauche (503 cm−1) and trans (461 cm−1) C2F2H4, respectively. An overall potential function describing the torsional barriers and ΔH will be discussed on the basis of the variable temperature data and the low frequency torsional transitions. The present study is consistent with an earlier electron diffraction study but disagrees with the conclusions advanced in a previous vibrational investigation of C2F2H4. These results will be reviewed in detail since the existent data is consistent with a predominant gauche conformer.

The structures and fragmentation pathways of two isomeric organophosphorus esters, dimethyl methylphosphonate (DMMP) and trimethyl phosphite (TMP) have been determined. The long-lived, low-energy molecular ions of DMMP were fonnd to undergo a keto-to-enol isomerhation prior to collision-induced dissociation. This isomerization was established through the comparison of the collision spectra from DMMP, TMP, isotopicaUy labeled DMMP and a model precursor ion. Electron ionization and charge exchange reactions were used to study the isomerization as a function of the internal energy of the molecular ion. The structure of the TMP molecular ion retained the structure of the neutral molecule. The daughter ion spectra of the isomeric fragment ions from DMMP and TMP were used to infer the fragment ion structures. Negative ions of DMMP and TMP were also studied, and their collision spectra were found to be indistinguishable.

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.

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