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 lev els 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/mo l. Our observations are at variance with an earlier ab initio calculat ion employing smaller basis sets, STO-3G and 4-31G*, which had report ed that the C-1 conformer was the lowest in energy. Furthermore, the e arlier calculation did not report the occurrence of a minimum correspo nding to the C-s symmetry. Vibrational frequency calculations were don e at the HF and MP2 levels. The computed frequencies were found to com pare well with experimental vapor phase and matrix isolation data repo rted earlier, leading to a definitive assignment of the infrared featu res 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 MP 2/6-31G* 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.