Structural and spectral consequences of ion pairing: A theoretical study of CF₃O^-Li⁺ and CF₃O^-Na⁺

Ion pairs CF₃O^-Li⁺ and CF₃O^-Na⁺ are predicted, by ab initio MO calculations at the 3-21G level of SCF theory, to be stable with respect to dissociation or decomposition and to prefer bidentate structures. The essential features of the infrared spectrum of triflouromethoxide anion are well reproduced by the calculations, particularly the ion-pairing induced splitting of degenerate modes as observed for matrix-isolated, as compared with crystalline, CF₃O^- Cs⁺. Polarization of the anion by the cation in a bidentate interaction is the major cause of these spectral splittings; the role of covalency is minor and diminishes as the size of the cation increases. Structural and spectral consequences of ion pairing are accounted for simply by changes in hyperconjugative delocalization of oxygen lone-pair electron density into CF-antibonding orbitals.