Theoretical study of CF3SO3Li, (CF3SO2)(2)NLi, and (CF3SO2)(2)CHLi ion pairs

Ion pairs of lithium salts based on three organic anions, namely Tf(-), TFSI-, and TFSM(-), have been studied by ab initio quantum-chemical methods. These calculations allowed us to draw the conformational energy diagram for TFSM(-) and TFSI-. Geometries and binding energies have been evaluated at the restricted Hartree-Fock (RHF) level using standard basis set (3-21+G* and 6-31+G*). For the reference Tf(-)-Li+, electron con-elation effects have been determined at the MP2 level. Wave function analyses have been performed by the natural bond orbital (NBO) method. Bidentate structures involving two oxygen atoms (one of each SO2 group in the TFSI-,Li+ and TFSM(-),Li+ systems) are preferred. The affinities of these anions for the lithium cation are strong and lie in the range 120-145 kcal . mol(-1), arising essentially from the electrostatic interaction. Charge transfer contributions are less important, only 14-17% of the electrostatic interaction.