MOLECULAR-STRUCTURES AND NORMAL VIBRATIONS OF CF3SO3- AND ITS LITHIUM ION-PAIRS AND AGGREGATES

The molecular structures, harmonic vibrational frequencies, infrared absorption intensities, internal force constants, and electronic charges of the trifluoromethanesulfonate (triflate) anion, its lithium ion pairs, and several aggregates have been studied using the ab initio self-consistent field Hartree-Fock method with 3-21 +G*, 6-31+G augmented with polarization functions (d type orbitals) on the sulfur atom, and 6-31+G* basis sets. The calculated frequency shifts of the upsilon(s)(SO3), upsilon(s)(CF3), delta(s)(CF3), and upsilon(CS) modes and the splitting widths of the upsilon(as)(SO3) modes in lithium triflate ion pairs and aggregates are compared with those observed in IR and Raman spectra of lithium triflate dissolved in some polar, aprotic solvents. A bidentate coordination of Li+ by triflate oxygens is the most stable structure for the isolated Li-Tf ion pairs, but a monodentate structure is apparently favored for Li.Tf ion pairs in polar, aprotic liquid solutions. Triflate ions in aggregates I and II of the lithium triflate are likely to have the bidentate bridging structure and the tridentate bridging structure, respectively. The calculated frequency shifts and splittings for the monodentate ion pair and bidentate bridging aggregate agree with the experimentally observed values for lithium triflate complexes in polar, aprotic liquids much better when each lithium cation is capped with an OH- group to model the effect of lithium coordination by solvent.