Nuclear magnetic resonance and molecular orbital study of phenylphosphine and some dihalogeno derivatives

Abstract

Ab initio(STO-3G) molecular orbital calculations, with partial geometry optimization, yield values of 1.12, 0.53 and 2.30 kcal mol^–1 for the barriers to internal rotation in phenylphosphine, phenyldifluorophosphine and phenyldichlorophosphine, respectively. The —PH₂ and —PF₂ substituents are predicted to have ground state conformations in which the lone pair lies perpendicular to the aromatic plane, but for —PCl₂ the conformer in which the lone pair in the plane is given as most stable. The inversion barriers for PhPH₂, PhPF₂, and PhPCl₂, are computed to be 56.3, 91.5 and 75.2 kcal mol^–1. The proton magnetic resonance spectra of PhPH₂, PhPCl₂ PhPF₂ and some ring chlorinated derivatives are fully analysed. The proton-coupled carbon-13 n.m.r. spectra of PhPCl₂ are analysed and the proton-decoupled spectra of PhPH₂ and PhPF₂ are reported. The chemical shift of the para-carbon is used to derive values of 0.06, 0.70 and 0.61 for the σ⁺_p constants of the —PH₂, —PF₂ and —PCl₂ substituents.

The long-range spin–spin coupling constants over four, five and six bonds, between sidechain protons (or fluorines) and ring protons, are shown to have the sign sequence –+–. INDO-FPT calculations are used to investigate the angular dependence of these long-range couplings, and also of the couplings from sidechain protons (or fluorines) to ring carbons. It is suggested that the five-bond couplings, ⁵J^C,H and ⁵J^C,F, to the para-carbon, are dominated by a σ–π mechanism. The couplings, ⁶J^{H H}_p and ⁶J^H,F_p are indicated as having a sin²θ dependence. It is shown that the geometry of the —PH₂ and —PF₂ groups precludes the use of the latter couplings to determine internal rotation barriers.

It is pointed out that both phosphorus–ring proton and phosphorus–ring carbon coupling constants in these phosphines increase in the order H < F < Cl, and that the phosphorus–ring proton couplings are strongly substituent dependent. Calculation of phosphorus couplings by the INDO-FPT method are found to be in poor agreement with experiment.