Inversion Processes in Phosphines and Their Radical Cations:  When Is a Pseudo-Jahn−Teller Effect Operative?^†

The inversion processes in substituted phosphines (R₃P) and their radical cations have been investigated in detail using high-level ab initio molecular orbital and density functional theory calculations. Particular attention has been paid to the understanding of the inversion mechanism. While in PH₃ and methyl derivatives only the classical vertex inversion exists, halogenated phosphines invert through C_2v T-shaped transition structures in most neutral species. The pseudo-Jahn−Teller effect in which an a₁‘ ⊗ e‘ mixing leads a D_3h to a C_2v T-shaped transition structure is confirmed to be responsible for the edge inversion. This effect is operative upon substitution of, at least, two H atoms of PH₃ by halogen atoms and appears to be largest in F-derivatives. The pseudo-JT effect is reduced in the order F > Cl > Br. In radical cations, the inversion barriers are consistently and substantially reduced, in such a way that in PCl₃^•+ and PBr₃^•+ the pseudo-Jahn−Teller effect virtually disappears and the edge inversion no longer exists. The effect of electron correlation on the inversion barriers has also been considered; it is small in hydrides but becomes quite large in halogenated phosphines. The ionization energies of the phosphines under consideration have also been derived.