Conformation, barrier to internal rotation, and structure of the PH 2-group in phenylphosphine, studied by microwave spectroscopy

Abstract

The microwave spectra of phenylphosphine (C 6H 5PH 2) and of mono- and dideuterated phenylphosphine (C 6H 5PHD and C 6H 5PD 2) were investigated. The two torsional sublevels of the vibrational ground state were assigned for each molecular species. Only μ a -and μ b -lines were observed, the μ b -lines being split by approximately twice the splitting of the torsional sublevels. By means of internal rotation calculations with a program that determines the effective Hamiltonian for one top molecules without symmetry restrictions, the twofold barriers to internal rotation were determined to be 94.2 cm −1 (C 6H 5PH 2), 91.1 cm −1 (C 6H 5PHD), and 85.5 cm −1 (C 6H 5PD 2). No effects due to inversion of the PH 2 group were observed in the spectra. For C 6H 5PH 2 and C 6H 5PD 2 the internal rotation potential is of the cos 2γ type with the minimum position corresponding to the configuration with the hydrogen atoms of the phosphine group on opposite sides of the phenyl ring plane, which is thus a plane of symmetry for the molecule. The C 6H 5PHD potential, however, contains an additional sin 2γ term with the effect of rotating the minimum 8.5° in the direction from D toward H. The direct monosubstitution method for determination of the hydrogen atoms of the PH 2 group gave inconsistent results, but the disubstitution method yielded a result comparable to the internal rotation calculation. The structural parameters determined for the PH 2 group are: PH = 1.414 Å, HPH = 94.27°, CPH = 97.29°, and tilt angle (CP to internal rotation axis) = 2.96° (P and H being on opposite sides of the internal rotation axis).