Large atomic orbital basis SCF and MRD-CI calculations for rearrangements of the PH5 molecule

Abstract

Configuration interaction calculations in a large atomic orbital basis are reported for PH₅ in order to study the relative merits of the Berry pseudo-rotation (BPR) and turnstile rotation (TR) mechanisms for ligand permutation in phosphoranes. A thorough geometrical optimization for the key trigonal bipyramid (D_3h), square pyramid (C_4v) and C_2v-C_3v TR (C_s) structures is effected in each case and reaction surfaces connecting these PH₅ isomers are constructed. The barrier for the BPR mechanism is only 1.0 kcal (with the C_4v structure corresponding to a potential maximum), while that for the TR process is 8.2 kcal if no relative tilting of the PH₂(C_2v) and PH₃(C_3v) portions of the molecules is allowed during ligand rotation. A tilted TR structure lying only 3.2 kcal above the optimal D_3h conformation is identified, however, and the possible significance of this result is discussed. Speculation concerning which of the BPR and TR mechanisms is preferred for rearrangements of substituted phosphoranes is also made and it is argued that the TR is most likely to be favoured when bulky and relatively electropositive ligands are present in the system.