Phosphonitrilic derivatives. Part XVIII. Ionisation potentials, orbital symmetry, and π-electron interactions

Abstract

The ionisation potentials of the phosphonitrilic fluorides (NPF₂)_n(n= 3–8) have been measured by photoelectron spectroscopy, and those of other phosphonitrilic derivatives (NPX₂)_n[X = Cl, n= 3–7; X = OCH₂·CF₃, OMe, OPh, NMe₂, or Me, n= 3 or 4] by electron impact. All the 6-membered cyclic compounds have first ionisation potentials which are higher than those of their 8-membered analogues, and in the fluorides the first ionisation potentials alternate as the size of the ring increases beyond 8 atoms. These results, and also the relative energies of a number of states of (NPF₂)_n⁺, are interpreted by use of the orbital approximation and a model of a dual π-system which involves 3d-orbitals at phosphorus and valence-shell orbitals of nitrogen and which supplements the underlying σ-framework. Comparison of the results with simple molecular orbital calculations suggest that the highest π-system is of the homomorphic type, involving mainly the 3d_x²–y^2– orbital, and that it is accompanied by a lower and partly overlapping heteromorphic system, to which the 3d_xz-orbital makes the major contributions. Ionisation spotential are also assigned to molecular orbitals which span the P–N and P–F σ-bonds, and the fluorine lone pairs.