JAHN-TELLER EFFECT IN $VCl_{4}$

Abstract

The Jahn-Teller effect on the $^{2}E$ ground state of $VCl_{4}$ has been investigated using ab initio restricted Hartree-Fock, Cl calculations, and spin-orbit Cl calculations with relativistic core potentials and gaussian double zeta plus polarization basis sets. The effects of the Jahn - Teller active E vibration were studied by computing the energy when opposite angles were simultaneously changed. The potential energy surface is described by $W =W(T_{d})+\frac{1}{2}kr^{2}\pm \left(ar+\frac{1}{2}k^{\prime}r^{2}\cos(3\phi)\right)$ where r and $\phi$ are the vibrational coordinates in polar form, and k, a, and $k^{\prime}$ are potential constants. At the SCF level, the minimum energy occurs with opposite angles opened by $3.5^{\circ}$, and is $155.8 cm^{-1}$ below the tetrahedral energy. Closing these angles by $3.3^{\circ}$ corresponds to a saddle point which is $4.5 cm^{-1}$ higher in energy. The force constant k and the first-order vibronic interaction constant a are calculated to be $0.124 mdyn/{\AA}$ and $2.75 \times 10^{-5}$ dyn respectively. At the CI level, the minimum energy occurs with opposite angles opened by $2.8^{\circ}$, and is $97.4 cm^{-1}$ below the tetrahedral energy. Closing these angles by $2.6^{\circ}$ corresponds to a saddle point on the surface which is $4.3 cm^{-1}$ higher in energy. the force constant k and the first-order vibronic interaction constant a are calculated to be $0.121 mdyn/{\AA}$ and $2.15 \times 10^{-5}$ dyn respectively. These values agree well with those found by electron $diffraction.^{1}$ The quadratic vibronic interaction constant $k^{\prime}$ is calculated to be $35.3 cm^{-1}/{\AA}$ and has not been measured experimentally. Initial spin-orbit Cl calculations show that the spin-orbit interaction has little effect on the adiabatic potential energy surface.