The preferred conformation(s) of trimethyl phosphate as derived from NMR spectra of partially oriented molecules and potential energy calculations

Abstract

The ¹H and ³¹P NMR spectra of oriented trimethyl phosphate have been investigated. Potential energy calculations on the system indicate that the gauche (with the rotation angle φ around the C---P bond(s) =60°) and the trans (with φ=155°) are the preferred conformers with the gauche having lower energy than the trans. Assuming reasonable bond lenghts and bond angles and similar order parameters of the threefold symmetry axis for the two preferred conformers, it is shown from NMR studies that the gauche conformer is predominant. Solvent/temperature dependence of the conformer ratio is evident from the NMR spectra. The ¹H and the ³¹P chemical shift anisotropies are determined from the spectra in mixed liquid crystals of opposite diamagnetic anisotropies.