Ab initio calculation of the structural parameters, conformational stability, vibrational frequencies and Raman and infrared intensities of methoxydimethylphosphine

Abstract

The structural parameters of the near-cis (where the methyl moiety of the methoxy group is nearly eclipsing the lone pair on the phosphorus atom) and the trans (methoxymethyl with a dihedral angle of 180° from the phosphorus lone pair) conformations of methoxydimethylphosphine, CH 3OP(CH 3) 2, have been calculated with the RHF/3-21G ∗ and RHF/6-31G ∗ basis sets. For the smaller basis set the trans conformer is calculated to be more stable by 128 cm −1 (366 cal mol −1) than the near-cis rotamer, whereas with the 6-31G ∗ basis set the near-cis conformer is more stable than the trans conformer by 549 cm −1 (1570 cal mol −1). This latter result is consistent with the previously reported vibrational data for the vapor where predominately one conformer was observed. With the 6-31G ∗ basis set the near-cis rotamer has the methoxymethyl group rotated by 20° from eclipsing the phosphorus atom lone pair with a near-cis to near-cis barrier of 10 cm −1. Full optimization was also performed at the near-cis/trans transitional state and the barrier was calculated to be 1405 cm −1 (4.02 kcal mol −1) and 1675 cm −1 (4.77 kcal mol −1) with the 3-21G* and 6-31G* basis sets respectively. The force constants, vibrational frequencies, potential energy distributions, and the IR and Raman intensity parameters for both stable conformers were calculated with the 3-21G* basis set. These data are used to provide definitive assignments for the normal modes for both stable confonners.