Lithium Ion Solvation in Room-Temperature Ionic Liquids Involving Bis(trifluoromethanesulfonyl) Imide Anion Studied by Raman Spectroscopy and DFT Calculations

The solvation structure of the lithium ion in room-temperature ionic liquids 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl) imide (EMI+TFSI-) and N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl) imide (BMP+TFSI-) has been studied by Raman spectroscopy and DFT calculations. Raman spectra of EMI+TFSI- and BMP+TFSI- containing Li+TFSI- over the range 0.144−0.589 and 0.076−0.633 mol dm-3, respectively, were measured at 298 K. A strong 744 cm-1 band of the free TFSI- ion in the bulk weakens with increasing concentration of the lithium ion, and it revealed by analyzing the intensity decrease that the two TFSI- ions bind to the metal ion. The lithium ion may be four-coordinated through the O atoms of two bidentate TFSI- ions. It has been established in our previous work that the TFSI- ion involves two conformers of C 1 (cis) and C 2 (trans) symmetries in equilibrium, and the dipole moment of the C 1 conformer is significantly larger than that of the C 2 conformer. On the basis of these facts, the geometries and SCF energies of possible solvate ion clusters [Li(C 1-TFSI-)2]-, [Li(C 1-TFSI-)(C 2-TFSI-)]-, and [Li(C 2-TFSI-)2]- were examined using the theoretical DFT calculations. It is concluded that the C 1 conformer is more preferred to the C 2 conformer in the vicinity of the lithium ion.