SPECTRA AND STRUCTURE OF ORGANOPHOSPHORUS COMPOUNDS .51. IR AND RAMAN-SPECTRA, CONFORMATIONAL STABILITY, BARRIERS TO INTERNAL-ROTATION, VIBRATIONAL ASSIGNMENT, AND AB-INITIO CALCULATIONS OF N-PROPYLPHOSPHINE

The Raman (3100 to 50 cm-1) and IR (3100 to 50 cm-1) spectra of gaseou s and solid n-propylphosphine, C3H7PH2, and the corresponding P-d2 iso topomer have been recorded. Additionally, the Raman spectra of the liq uids have been obtained with qualitative depolarization ratios. From t hese data, all five possible conformers have been identified in the fl uid states and the trans-trans conformer is shown to be the most stabl e rotamer in both the gaseous and liquid states and it is the only con former present in the solid. The first trans refers to the orientation of the lone pair to the ethylene group (rotation around the P-C bond) whereas the second trans refers to the orientation of the methyl grou p relative to the P-C bond (rotation around the -CH2-CH2 bond). The ne xt most stable conformer is the gauche-trans rotamer where the enthalp y difference has been determined from variable-temperature Raman studi es to be 140 +/- 5 cm-1 (400 +/- 14 cal mol-1) for the vapor and 351 /- 20 cm-1 (1004 +/- 57 cal mol-1) for the liquid. The other three con formers have nearly the same stabilities but significantly higher ener gies than the two more stable rotamers. From the far-IR data and relat ive conformer stabilities, some of the coefficients of the potential f unction governing conformer interconversion are estimated. A complete vibrational assignment is proposed for the trans-trans conformer and f or the fundamentals for most of the heavy atom motions for the other c onformers. The conformational stabilities, barriers to internal rotati on, and fundamental vibrational frequencies which have been determined experimentally are compared to those obtained from ab initio calculat ions employing the RHF/3-21G and/or RHF/6-31G* basis sets. Additional ly, the conformational stabilities and structural parameters have been determined with the 6-31G basis set with electron correlation at the MP2 level. These results are compared with the corresponding quantiti es for some similar molecules.