Valence state of a heteroatom and the conjugation problem

Abstract

Calculations of the electronic structure and geometry ofs-cis-, s-trans- andgauche-conformers of AllPH₄ (1) molecule (All = CH₂CHCH₂) were carried out by the quantum chemicalab initio SCF MO LCAO method with the 3-21G basis. The results obtained were compared to those found earlier for AllPH₂ (2) and AllPF₄ (3). The increase in stability of tilegauche-conformer relative to thetrans-conformer on going from2 to1 is due to the significant contribution of vacant d-orbitals of the P atom in the LUMO, to the increase in interactions of frontier MOs through space, and (to a lesser degree) through bond interaction with the CH₂ bridging group, as well as to transfer of electron density from an allyl fragment. The conjugation in allylphosphines is determined by localization of the HOMO, which correlates with the lone electron pair of the P atom in2 and with σ-MO in1; it should be enhanced in allylphosphines containing the axial P-C bond.