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 (trif late) ion pair, CF3SO3-Li have been investigated using the ab initio s elf-consistent Hartree-Fock and correlated second-order Moller-Plesset perturbation theory with the 6-31G and lower basis sets. In the opti mized structure the lithium cation is bound to two of the oxygens of t he SO3 group forming a bidentate complex with C(s) symmetry. A local m inimum with a monodentate structure was obtained in the HF/3-21G calc ulations. The energy difference between the mono- and bidentate struct ures of the complex is predicted to be nearly 39 kJ mol-1 in this basi s. A splitting of 230 and 158 cm-1 is obtained for the antisymmetric S O3 stretching for the bi- and monodentate coordination of the lithium cation with the free anion, respectively. The infrared spectrum of lit hium triflate in poly(propylene oxide) shows a splitting of 43 cm-1. T he strong interaction of the metal cation with the anion in the 1:1 co mplex thus overemphasizes the ''splitting behavior'' observed for lith ium triflate dissolved in polymers. In the bidentate (MP2/6-31G) comp lex the symmetric SO3 stretching shows a downshift of 38 cm-1, in cont rast to an upshift of 47 cm-1 for the monodentate complex. The differe nt 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.