Spectra and structure of organophosphorus compounds: Part XLIII. Far-infrared spectra and ab initio calculations of ethylphosphine

Abstract

The far-IR spectra (350 to 30 cm −1) of gaseous ethylphosphine and the P- d 2 derivative have been recorded at 0.1 cm −1) resolution. These data have been utilized to reinterpret and verify assignments to the methyl torsional fundamentals. The fundamental vibrational frequencies for both the more stable trans (methyl group oriented trans to the lone pair on the phosphorus atom) and high energy gauche rotamers are calculated by ab initio Hartree-Fock gradient calculations employing the 3–21G ∗ basis set and these frequencies are compared to those obtained experimentally. Potential surfaces for both the methyl and asymmetric torsional modes have been calculated utilizing both the 3–21G ∗ and 6–31G ∗ basis sets. The theoretically determined barriers for both torsional modes are compared to those determined from analysis of the torsional bands observed in the far-IR spectra. The optimized geometries for both conformations are compared to the structures obtained from microwave spectroscopy. It is concluded that at these levels of calculation the structure, potential surface and vibrational frequencies for ethylphosphine are well determined.