The Molecular Structure of Molybdenum Pentachloride Studied by Ab Initio Molecular Orbital Calculations and Gas Electron Diffraction.

Abstract

Molecular orbital calculations for highly symmetric conformers of gas-phase MoCl5 at three levels [all-electron restricted Hartree-Fock (RHF) calculations, relativistic effective core potential (RECP) RHF calculations and modified coupled pair functional RECP calculations] have been carried out. The calculations indicate that a D3h structure is more stable than a C4v configuration by 10-11 kcal mol-1. A distortion from D3h to C2v symmetry splits the orbitally degenerate 2E'' electronic state into 2B1 and 2A2 states, which are stabilized by 0.24 and 0.35 kcal mol-1, respectively, at the RECP RHF level, suggesting that the molecule may undergo a dynamic Jahn-Teller distortion. Gas electron-diffraction data of MoCl5 were best fitted by distorted trigonal bipyramidal models of C2v symmetry with large-amplitude motion. A simple dynamical model for the electron-diffraction data based on the ab initio results allowed the refinement of three structural parameters to yield good agreement with the electron-diffraction data. The results of the two methods agree if a relatively flat potential for the distortion towards the C4v form is assumed. Electron diffraction as well as ab initio calculations exclude a possible pseudorotation by the Berry mechanism over a C4v barrier.