Solvation Structure of Li⁺ in Methanol and 2-Propanol Solutions Studied by ATR-IR and Neutron Diffraction with ⁶Li/⁷Li Isotopic Substitution Methods

Neutron diffraction measurements have been carried out on 10 mol % LiTFSA (TFSA: bis(trifluoromethylsulfonil)amide) solutions in methanol- d₄ and 2-propanol- d₈ to obtain information on the solvation structure of Li⁺. The detailed coordination structure of solvent molecules within the first solvation shell of Li⁺ was determined through the least-squares fitting analysis of the difference function between normalized scattering cross sections observed for ⁶Li/⁷Li isotopically substituted sample solutions. The nearest-neighbor Li⁺···O distance and coordination number determined for the 10 mol % LiTFSA-methanol- d₄ solution are r_LiO = 1.98 ± 0.02 Å and n_LiO = 3.8 ± 0.6, respectively. In the 2-propanol- d₈ solution, it has been revealed that 2-propanol- d₈ molecules within the first solvation shell of Li⁺ take at least two different coordination geometries with the intermolecular nearest-neighbor Li⁺···O distance of r_LiO = 1.93 ± 0.04 Å. The Li⁺···O coordination number, n_LiO = 3.3 ± 0.3, is determined. Ion-pair formation in the LiTFSA-methanol and LiTFSA-2-propanol solutions has been investigated by the attenuated total reflection infrared spectroscopic method. Mole fractions of free, Li⁺-bound, and aggregated TFSA^- are derived from the peak deconvolution analysis of vibrational bands observed for TFSA^-.