CONFORMATIONAL STABILITY OF TRIMETHYLPHOSPHATE STUDIED BY VIBRATIONALSPECTROSCOPY AND AB-INITIO CALCULATIONS

The IR and Raman spectra of the trimethylphosphate (TMP) were recorded in the vapour, liquid and crystalline states and as a solute in solve nts of different polarity. The crystalline phases were recorded, and I R spectra in argon and nitrogen matrices at 5-14 K were obtained with the hot nozzle technique. Due to restricted rotations around the three P-O bonds, TMP can exist in many different conformers with symmetries : C-3v, C-3 and C-1. Our ab initio calculations carried out with Gauss ian 90, using the basis set 6-31 G predict three dihedral angles of O =P-O-C all equal to 44.80 (C-3 symmetry) and one conformer with C-1 sy mmetry having dihedral angles equal to 34.7, 50.3 and 178.2 degrees). The energy difference Delta H-0(C-3-C-1) was calculated to be 4.2 kJ m ol(-1). Small spectral changes in variable temperature Raman measureme nts and in IR matrix experiments at different nozzle temperatures sugg est two stable conformers with a small energy difference. Because of t he large dipole moment of the C-1 conformer compared with C-3, the for mer has increased stability in polar solvents. Two crystal modificatio ns were observed (one obtained without proper annealing and probably m etastable), the other achieved after annealing: both contains TMP in t he C-1 conformer.