GEOMETRY AND VIBRATIONAL FREQUENCIES OF THE LITHIUM TRIFLATE ION-PAIR - AN AB-INITIO STUDY

The optimized geometry, harmonic vibrational frequencies, and infrared absorption intensities of the lithium trifluoromethanesulfonate (triflate) ion pair, CF3SO3-Li have been investigated using the ab initio self-consistent Hartree-Fock and correlated second-order Moller-Plesset perturbation theory with the 6-31G* and lower basis sets. In the optimized structure the lithium cation is bound to two of the oxygens of the SO3 group forming a bidentate complex with C(s) symmetry. A local minimum with a monodentate structure was obtained in the HF/3-21G* calculations. The energy difference between the mono- and bidentate structures of the complex is predicted to be nearly 39 kJ mol-1 in this basis. A splitting of 230 and 158 cm-1 is obtained for the antisymmetric SO3 stretching for the bi- and monodentate coordination of the lithium cation with the free anion, respectively. The infrared spectrum of lithium triflate in poly(propylene oxide) shows a splitting of 43 cm-1. The strong interaction of the metal cation with the anion in the 1:1 complex thus overemphasizes the ''splitting behavior'' observed for lithium triflate dissolved in polymers. In the bidentate (MP2/6-31G*) complex the symmetric SO3 stretching shows a downshift of 38 cm-1, in contrast to an upshift of 47 cm-1 for the monodentate complex. The different signs of these frequency shifts have a purely geometric origin. The dependence of this frequency shift on the position of the Li+ ion is discussed.