Conformational stability of trimethylphosphate studied by vibrational spectroscopy and ab initio calculations

Abstract

The IR and Raman spectra of the trimethylphosphate (TMP) were recorded in the vapour, liquid and crystalline states and as a solute in solvents of different polarity. The crystalline phases were recorded, and IR 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: C3v, C3 and C1. Our ab initio calculations carried out with Gaussian 90, using the basis set 6–31 G∗ predict three dihedral angles of O=P-O-C all equal to 44.8° (C3 symmetry) and one conformer with C1 symmetry having dihedral angles equal to 34.7, 50.3 and 178.2°). The energy difference ΔHo(C3-C1) was calculated to be 4.2 kJ mol−1. Small spectral changes in variable temperature Raman measurements and in IR matrix experiments at different nozzle temperatures suggest two stable conformers with a small energy difference. Because of the large dipole moment of the C1 conformer compared with C3, the former has increased stability in polar solvents. Two crystal modifications were observed (one obtained without proper annealing and probably metastable), the other achieved after annealing: both contains TMP in the C1 conformer.