Intermolecular Self-Interactions of the Titanium Tetrahalides TiX<sub>4</sub> (X = F, Cl, Br)

1999-03-04T00:00:00Z (GMT) by Simon P. Webb Mark S. Gordon
Ab initio calculations have been performed on the closed-shell molecules TiX<sub>4</sub> and Ti<sub>2</sub>X<sub>8</sub> (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. Ti<sub>2</sub>F<sub>8</sub> is predicted to be a bound <i>C</i><sub>2</sub><i><sub>h</sub></i> dimer with bridging bonds, lower in energy than the separated monomers by 10.5 kcal/mol. Ti<sub>2</sub>Cl<sub>8</sub> and Ti<sub>2</sub>Br<sub>8</sub> are predicted to be weakly bound dimers whose structures are that of associated monomers with overall <i>D</i><sub>3</sub><i><sub>d</sub></i> point group symmetry. Ti<sub>2</sub>Cl<sub>8</sub> 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 TiCl<sub>4</sub> and in mixed systems such as TiCl<sub>4</sub>/TiBr<sub>4</sub>.