Non-empirical calculations of the electronic structure of some five-membered ring heterocycles containing sulphur and phosphorus: thiophen and phosphole

Abstract

A comparison of the electronic structures of thiophen and phosphole and their first row analogues furan and pyrrole has been made by a linear combination of gaussian orbital (LCGO) procedure. The LCGO procedure indicates a non-planar structure for phosphole itself, as found for the 1-benzyl derivative; this arises from an increase in 3S_P orbital contributions to the bonding in the pyramidal case. The low inversion barrier in phospholes has been interpreted by a valency force field, without reference to aromatic character. In both molecules the 3d orbitals behave as polarisation functions rather than strongly bonding functions. Phosphole has no resonance energy, and the π-levels 3b₁ and 1a₂ are reversed in binding energy order relative to thiophen. The ¹H n.m.r. spectra of simple phospholes have been compared with those of cyclopentadiene, rather than of thiophen.