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

摘要

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 Li-6/Li-7 isotopically substituted sample solutions. The nearest-neighbor Li+center dot center dot center dot O distance and coordination number determined for the 10 mol % LiTFSA-methanol-d(4) solution are r(LiO) = 1.98 +/- 0.02 angstrom and nuo = 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+center dot center dot center dot O distance of r(LiO) = 1.93 +/- 0.04 angstrom. The Li+center dot center dot center dot 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(-).