DETERMINATION OF THE STRUCTURE AND CHIROPTICAL PROPERTIES OF THE PARENT NERVE-GAS O-METHYL METHYLPHOSPHONOFLUORIDATE BY AB-INITIO CALCULATIONS, ELECTRON-DIFFRACTION ANALYSIS, AND NMR-SPECTROSCOPY

The geometry and electronic structure of O-methyl methylphosphonofluoridate (1) (MeOPOFMe) have been determined at the MP2/6-31G(D) and Becke3LYP/6-31G(D) levels. The calculations confirm the prior results of lower-level ab initio calculations which indicated that the experimental gas phase electron diffraction structure of 1 is substantially in error, The compound exists in a single conformation determined by the operation of an anomeric effect between the methoxy oxygen atom and the P-F bond as is evident from the nearly perpendicular orientation of the O-C and P-F bonds. A reanalysis of the published gas electron diffraction (GED) data yielded inconclusive results but underscored the inability of GED to distinguish between the conformations of 1. H-1 and C-13 NMR spectra are reported and are consistent with the existence of a single conformer. The lower excited singlet states of 1 are calculated to be Rydberg states originating from excitations from the p-type nonbonding orbitals of-the monocoordinated oxygen atom into diffuse 3s- and 3p-type orbitals. The transitions are predicted to occur in the vacuum UV and have low oscillator strength. The first two transitions have moderate positive rotatory strength for the (R) enantiomer, leading to the prediction of absolute configuration, (+)-(R)-1.