TRENDS IN INVERSION BARRIERS OF GROUP-15 COMPOUNDS .2. AB-INITIO AND DENSITY-FUNCTIONAL CALCULATIONS ON GROUP-15 FLUORIDES

Two-dimensional hypersurfaces for the inversion process of the group 1 5 fluorides MF(3) (M = N, P, As, Sb, Bi) involving the MF(3) umbrella motion and the symmetric F-M-F bending mode have been calculated at th e Hartree-Fock level. The MO scheme of NF3 shows an a(2)'' HOMO and a( 1)' LUMO in accordance with the frontier orbitals of the group 15 hydr ides, and therefore it inverts through the expected trigonal planar D- 3h transition state. For PFS, AsF3, SbF3, and BiF3, however, the HOMO- LUMO sequence is inverted and the HOMO is of a(1)' symmetry. This resu lts in an a(1)'(xe)(HOMO)'(LUMO) mixing at the trigonal planar D-3h st ructure causing a second-order Jahn-Teller symmetry breaking (distorti on Hamiltonian is of e' symmetry) toward the T-shaped C-2v arrangement . The minimum energy inversion path is of C-s symmetry starting from t he pyramidal C-3v minimum structure by distorting immediately toward t he T-shaped C-2v first-order transition state. Moller-Plesset second-o rder results (MP2) are compared with local density functional approxim ation (LDA) methods including nonlocal corrections and several semiemp irical (AM1, PM3, and MNDO) calculations. The performance of different exchange and correlation functionals within LDA including gradient co rrections is discussed in detail. Because of the a(1)'(xe)(HOMO)'(LUMO ) mixing, electron correlation contributions become very important and Freed's theorem is not valid for the fluorides of the heavier element s P, As, Sb, and Bi. Hence, to assess the validity of a single-determi nant MP2 approach we performed complete active space MP2 calculations (CASPT2) for the minimum and inversion structures of NF3 and PF3. Thes e calculations show that for PF3 a multireference scheme is necessary to describe the energetics of the D-3h to C-2v Jahn-Teller distortion accurately. The inversion barrier from the C-3v minimum structure to t he C-2v inversion point is, however, satisfactorily described by a sin gle-reference MP2 procedure. Vibrational frequencies and structure are predicted for gas-phase BiF3 at the MP2 level. The relativistic chang e in the F-Bi-F bond angle is large (alpha(NR) - alpha(R) = -1.5 degre es at the MP2 level), resulting in an anomaly in the trend of F-M-F bo nd angles along the group 15 fluorides. When the vibrational frequenci es are considered, MP2 of all the methods performs the best while, som ewhat surprisingly, the Becke correction to the LDA exchange functiona l leads to large errors. Despite this, the LDA inversion barriers seem to be more accurate than the single-reference MP2 values, and in the case of PF3 both the LDA (using the Vosko-Wilk-Nusair (VWN) parametriz ation) and the CASPT2 method are in close agreement. For BiF3 at the r elativistic level, LDA/VWN and PM3 predict a trigonal planar inversion structure contradicting MP2 and coupled cluster results. However, if relativistic effects are neglected, both LDA and MP2 yield a T-shaped inversion state.