INFRARED AND RAMAN SPECTRA, CONFORMATIONAL STABILITY, BARRIERS TO INTERNAL ROTATION, VIBRATIONAL ASSIGNMENT AND AB INITIO CALCULATIONS OF N-PROPYLPHOSPHINE
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Date
1993
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Publisher
Ohio State University
Abstract
The Raman (3100 to $50 cm^{-1}$) and infrared (3100 to $50 cm^{-1}$) spectra of gaseous and solid n-propylphosphine, $C_{3}H_{7}PH_{2}$, and the corresponding $P-d_{2}$ isotopomer have been recorded. Additionally, the Raman spectra of the liquids have been obtained with qualitative depolarization ratios. From these data, all five possible conformers have been identified in the fluid states and the trans-trans conformer is shown to be the most stable rotamer in both the gaseous and liquid states and it is the only conformer present in the solid. The first trans refers to the orientation of the lone pair with respect to the ethyl group (rotation around the P-C bond) whereas the second trans refers to the orientation of the methyl group relative to the P-C bond. The next most stable conformer is the gauche-trans rotamer where the enthalpy difference has been determined from variable temperature Raman studies to be $140 \pm 20 cm^{-1} (400 \pm 57$ cal/mol) for the vapor and $337 \pm 30 cm^{-1} (964 \pm 86 cal/mol)$ for the liquid. The other three conformers have nearly the same stabilities but significantly higher energies than the two more stable rotamers. From the far infrared data and relative conformer stabilities. some of the coefficients of the potential function governing conformer interconversion are estimated. A complete vibrational assignment is proposed for the trans-trans conformer as well as for the fundamentals for most of the heavy atom motions for the other conformers. The conformational stabilities, barriers to internal rotation, and fundamental vibrational frequencies which have been determined experimentally are compared to those obtained from ab initio calculations employing the $RHF/3-21G^{\ast}$ and $RHF/6-31G^{\ast}$ basis set. Additionally, the conformational stabilities and structural parameters have also been determined with the $6-31G^{\ast}$ basis set with electron correlation at the MP2 level. These results are compared to the corresponding quantities for some similar molecules.
Description
Author Institution: Department of Chemistry and Biochemistry, University of South Carolina; Department of Chemistry, Teachers College, Kyungpook National University