Intermolecular Self-Interactions of the Titanium Tetrahalides TiX₄ (X = F, Cl, Br)

Ab initio calculations have been performed on the closed-shell molecules TiX₄ and Ti₂X₈ (X = F, Cl, Br) in order to determine the magnitude and the nature of the intermolecular self-interactions of the titanium tetrahalides. Geometry optimizations have been carried out using an effective core potential basis set with polarization, including the effects of dynamic electron correlation through second-order perturbation theory (MP2). The importance of higher order correlation effects is examined through coupled cluster single-point energy calculations. Basis set effects are investigated using MP2 single-point energy calculations with large all-electron basis sets. Ti₂F₈ is predicted to be a bound C_2h dimer with bridging bonds, lower in energy than the separated monomers by 10.5 kcal/mol. Ti₂Cl₈ and Ti₂Br₈ are predicted to be weakly bound dimers whose structures are that of associated monomers with overall D_3d point group symmetry. Ti₂Cl₈ is lower in energy than separated monomers by 4.9 kcal/mol. Transition states have been found that represent paths to halide exchange between monomers supporting experimental evidence for rapid halide exchange in liquid TiCl₄ and in mixed systems such as TiCl₄/TiBr₄.