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-d 4 and 2-propanol-d 8 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-d 4 solution are r LiO = 1.98 ± 0.02 Å and n LiO = 3.8 ± 0.6, respectively. In the 2-propanol-d 8 solution, it has been revealed that 2-propanol-d 8 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–.