Nuclear magnetic resonance and molecular orbital study of phenylphosphine and some dihalogeno derivatives
- 1 January 1978
- journal article
- research article
- Published by Royal Society of Chemistry (RSC) in Journal of the Chemical Society, Faraday Transactions 2: Molecular and Chemical Physics
- Vol. 74, 933-950
- https://doi.org/10.1039/f29787400933
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, 5JC,H and 5JC,F, to the para-carbon, are dominated by a σ–π mechanism. The couplings, 6JH H p and 6JH,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.Keywords
This publication has 0 references indexed in Scilit: