Trans-ethylphosphine and selected deuterated isotopomers: Prediction of infrared spectra and potential energy distribution determination

The normal mode frequencies of isotopomers of trans- and gauche-ethylphosphine and its deuterated species are determined using the gaussian 98 quantum chemistry code. The calculations were performed at the Hartree-Fock, second-order Møller-Plesset and density functional levels with the 6-311G** basis set. The trans conformer was more stable than the gauche conformer by 2.24 kJ/mol at the DFT level theory. At 25°C, ΔG⁰ for the trans→gauche equilibrium is 2.06 kJ/mol and the relative population of gauche:trans conformers is 46.6%. The effect of deuterations of the hydrogen atoms in each functional group, separately and in combination, is studied. A comparison was made with the experimental infrared spectra for trans and gauche conformers of CH₃CH₂PH ₂ and CH₃CH₂PD₂. For the trans conformer of eight isotopomers of ethylphosphine, potential energy distributions were determined using the program MOLVIB and Kim's Correspondence Rules of unified group theory. The PED results were used to assign predominant vibrational motions for the trans conformer. The RMS deviation of calculated frequencies from experimental values is reported. The average scaling factors for all vibrational motions for CH₃CH₂PH₂ and CH₃CH₂PD₂ are 0.9197 for HF, 0.9534 for MP2 and 0.9860 for DFT. These average scaling factors are very similar to those previously determined for methylphosphine.