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

1998-07-18T00:00:00Z (GMT) by Steven Creve Minh Tho Nguyen
The inversion processes in substituted phosphines (R<sub>3</sub>P) 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 PH<sub>3</sub> and methyl derivatives only the classical vertex inversion exists, halogenated phosphines invert through <i>C</i><sub>2</sub><i><sub>v</sub></i> T-shaped transition structures in most neutral species. The pseudo-Jahn−Teller effect in which an a<sub>1</sub>‘ ⊗ e‘ mixing leads a <i>D</i><sub>3</sub><i><sub>h</sub></i> to a <i>C</i><sub>2</sub><i><sub>v</sub></i> 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 PH<sub>3</sub> 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 PCl<sub>3</sub><sup>•+</sup> and PBr<sub>3</sub><sup>•+</sup> 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.