Infrared absorption spectra of jahn-teller systems: Application to the transition-metal trifluorides MnF₃ and NiF₃

The theory for the calculation of vibronic absorption spectra within a Jahn-Teller (JT) active electronic state from first principles has been developed. The infrared absorption spectra of the ⁵E′ ground state, the low-lying ⁵E′ excited state of MnF₃, and the ⁴E′ state of NiF₃ have been computed and analyzed. Dipole moment derivatives have been determined by a linear-plus-quadratic expansion of nuclear dipole moment functions in the JT-active coordinates. Electronic transition dipole moments have been taken into account in the Condon approximation in the diabatic representation. The initial and final vibronic states have been expanded in a product of diabatic electronic states and vibrational basis functions. The effect of spin-orbit coupling on the vibronic infrared spectra of these molecules in their JT-active electronic states has been investigated, by employing the Breit-Pauli spin-orbit operator. The effect of temperature on the vibronic infrared spectra has also been explored. These results represent the first theoretical study of vibronic infrared spectra of JT-active states in transition metal compounds.