Solvation of Lithium Ion in N,N-Diethyl-N-methyl-N-(2-methoxyethyl)ammonium Bis(trifluoromethanesulfonyl)-amide Using Raman and Multinuclear NMR Spectroscopy

Abstract

The solvation structure of the Li(I) species in N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium bis(trifluoromethane-sulfonyl) amide (DEMETFSA) was studied by measuring the Raman and multinuclear NMR spectra of DEMETFSA solutions containing LiTFSA of various concentrations (0.12-1.92 mol kg(-1), [TFSA(-)]/[Li(I)] = 20.0-2.22). It was found from Raman spectra that an intense band due to the free TFSA(-) anion at around 741 cm(-1) becomes weak, and a new band appears at around 747 cm(-1) with an increase in the concentrations of LiTFSA, and that the pseudoisosbestic point is observed at around 744 cm(-1) in the range of [TFSA(-)]/[Li(I)] = 20.0-5.00. From analyses of these Raman bands, the number of TFSA(-) anions bound to the Li(+) ion was evaluated to be 1.85 +/- 0.08, and hence, the Li(I) in DEMETFSA solutions was proposed to exist as [Li(TFSA)(2)](-) in the range of [TFSA(-)]/[Li(I)] = 20.0-5.00. Furthermore, in the range of [TFSA(-)]/[Li(I)] = 2.86-2.22, the band observed at around 747 cm(-1) became more strong, and the pseudoisosbestic point disappeared. From these phenomena, it seems that the Li(I) oligomer species are formed in the higher concentration region of LiTFSA. The (19)F NMR signal of the TFSA(-) anion observed at 42.31 ppm in neat DEMETFSA was found to shift to a higher field linearly with an increase in the concentrations of LiTFSA ([LiTFSA] = 0.00-0.99 mol kg(-1), [TFSA(-)]/[Li(+)] = 20.0-3.33), while in a higher concentration range ([LiTFSA] > or = 1.26 mol kg(-1), [TFSA(-)]/[Li(+)] < or = 2.86), a slight deviation from linearity was observed. On the other hand, the (7)Li NMR signal did not show an appreciable shift with increasing LiTFSA concentrations. These results support that the Li(I) species in DEMETFSA solutions exist as [Li(TFSA)(2)](-) and the Li(I) oligomer species in the low and high concentration regions of LiTFSA, respectively.