Spectra and structure of organophosphorus compounds. LIII. Infrared and Raman spectra, conformational stability, vibrational assignment and ab initio calculations of n-propyldichlorophosphine

Abstract

The Raman (3100 to 50 cm −1) spectra of the liquid and solid and the infrared (3100 to 50 cm −1) spectra of the gaseous and solid n-propyldichlorophosphine, C 3H 7PCl 2, have been recorded. Additionally, the infrared (3100 to 400 cm −1) spectrum of the sample dissolved in liquid xenon has been obtained. Qualitative Raman depolarization ratios have been reported for the liquid phase. The spectra of the fluid phases have been interpreted on the basis of three out of five possible conformations. In the most stable trans-trans (TT) conformer, the phosphorus electron lone pair is in a trans position relative to the ethylene group (first trans) and the conformation of the heavy atom skeleton is also trans (second trans). The next most stable conformer is the gauche-trans (GT) rotamer where the enthalpy difference has been determined to be 1.68 ± 12 cm −1 (481 ± 35 cal mol −1) from a variable-temperature Raman study in the liquid phase and 105 ± 26 cm −1 (302 ± 75 cal mol −1) from a variable-temperature study of the infrared spectra of the sample dissolved in liquid xenon. Enthalpy differences have also been obtained for the gauche-I-gauche (G 1G) conformer which is third strongest in stability. In the solid phase, only the TT conformer has been identified. A complete vibrational assignment is proposed for the TT conformer as well as for some of the fundamentals of the other two more stable rotamers. The structural parameters, rotational constants, dipole moments and energies have been obtained from ab initio calculations employing the RHF/3-21G∗, RHF/6-31G∗ and MP2/6-31G∗ basis sets. The vibrational assignments are consistent with the calculated (RHF/3-21G∗ with two scaling factors) fundamental vibrational frequencies. These results are compared with similar quantities obtained for n-propylphosphine and ethyldichlorophosphine.