Spectra and structure of organophosphorus compounds. XIV. Infrared and Raman spectra, vibrational assignment and the asymmetric potential function for ethylphosphine and ethylphosphine-d2

Abstract

The infrared spectra of gaseous and solid CH₃CH₂PH₂ and CH₃CH₂PD₂ have been recorded from 200 to 3500 cm⁻¹. The Raman spectra of gaseous, liquid, and solid CH₃CH₂PH₂ and CH₃CH₂PD₂ have been recorded from 100 to 3500 cm⁻¹. The gauche and trans isomers have been observed in the fluid phases and only the trans exists in the solid state. A vibrational assignment of the 24 normal modes has been made. Fundamental and hot band transitions have been observed for both the methyl and phosphino torsions. The methyl barrier in the trans d₀ molecule was determined to be 3.74±0.05 kcal/mole and 3.14±0.05 for the gauche d₀ molecule. The potential function for internal rotation around the C–P bond in CH₃CH₂PD₂ was determined and the following potential constants found: V₂=207±6 cm⁻¹, V₃=785±5 cm⁻¹, and V₆=−25±3 cm⁻¹. The energy difference between the potential energy minima of the gauche and trans conformations is 150 cm⁻¹, the trans being the more stable form. The calculated gauche–gauche barrier is 635 cm⁻¹ (1.82 kcal/mole) and the trans–gauche barrier is 944 cm⁻¹ (2.70 kcal/mole). A similar potential function was determined for the CH₃CH₂PH₂ isomer but the fit to this one‐dimensional model was not as good as that for the deuterium compound probably because of greater coupling in the light molecule.