Spectra and structure of organophosphorus compounds. XXV—Raman and infrared spectra and conformational stability of ethyldimethylphosphine

Abstract

AbstractThe Raman (10–3500 cm−1) and infrared (50–3500 cm−1) spectra have been recorded for gaseous and solid ethyldimethylphosphine, CH3CH2P(CH3)2. Additionally, the Raman spectrum of the liquid has been recorded and qualitative depolarization values have been obtained. From the fact that several distinct Raman lines disappear on going from the fluid phases to the solid state, it is concluded that the molecule exists as a mixture of the gauche and trans conformers in the fluid phase with the gauche conformer being more stable and the only one present in the spectrum of the solid. From a temperature study of the Raman spectrum of the liquid, the enthalpy difference between the gauche and trans conformers was determined to be 134 ± 32 cm−1 (383 ± 92 cal mol−1). Relying on group frequencies and relative intensities of the infrared and Raman bands, and in some cases infrared band contours, a complete vibrational assignment is proposed for the gauche conformer. The assignment is supported by a normal coordinate calculation which was carried out utilizing a modified valence force field to obtain the frequencies of the normal modes and the potential energy distribution. The CH3‐P torsions have been observed at 208 and 185 cm−1 in the gas phase and from these frequencies the periodic barriers to internal rotation have been calculated to be 905 ± 95 cm−1 (2.56 kcal mol−1). The CH3‐C torsion was also observed in the gas phase at 217 cm−1 from which a periodic barrier of 1134 cm−1 (3.22 kcal mol−1) was calculated. The asymmetric torsional mode has been observed for the gauche conformer in both the infrared and Raman spectra of the gas at 91 cm−1 with evidence of ‘hot bands’ at lower frequencies. All of these results are compared with corresponding quantities for several other organophosphines.