Ab initio study of trimethyl phosphate: Conformational analysis, dipole moments, vibrational frequencies, and barriers for conformer interconversion

Ab initio molecular orbital calculations on trimethyl phosphate (TMP) were done using 6-31G* and 6-31G** basis sets, both at RHF and MP2 levels of theory. We located three minima corresponding to C-3, C-1, and C(')s symmetries, given in order of increasing energies. At the MP2/6-31G** level, the energy difference between the C-3 and C-1 conformers was 0.56 kcal/mol, while that between the C-3 and C-s was 1.43 kcal/mol. Our observations are at variance with an earlier ab initio calculation employing smaller basis sets, STO-3G* and 4-31G*, which had reported that the C-1 conformer was the lowest in energy. Furthermore, the earlier calculation did not report the occurrence of a minimum corresponding to the C-s symmetry. Vibrational frequency calculations were done at the HF and MP2 levels. The computed frequencies were found to compare well with experimental vapor phase and matrix isolation data reported earlier, leading to a definitive assignment of the infrared features of TMP. The barrier for conformer interconversion, C-1 <----> C-3, was also computed at the HF/6-31G** and MP2/6-31G** levels. At the MP2/6-31G** level the C-1 --> C-3 barrier was found to be 2.20 kcal/mol, while that for C-3 --> C-1 was 2.76 kcal/mol.