Surface enhancement Raman scattering from a silver electrode in a methyl acetate solution containing LiAsF6 and (CH3)4NBr

The structure of the solvated lithium cation in methyl acetate (MA) and in acetone (AC) solutions has been investigated using Raman spectroscopy. Evidence for contact ion pairing between Li⁺ and AsF₆^- is also presented for both solvents. The degree of association is less for AC solutions. Surface enhanced Raman scattering spectra of solvated lithium cation and tetramethylammonium ion (TMA⁺) in MA solutions adsorbed on a silver electrode have been measured and compared to the normal Raman spectra of the corresponding solution. Shifts in Raman frequencies for both Li(MA)₄⁺ and the C₄-N symmetric and asymmetric stretching modes from TMA⁺ have been observed. The results show that (CH₃)₄N⁺ adsorbs to a Ag electrode at low concentrations (5×10^-3 M) and positive potentials. Li(MA)₄⁺ also adsorbs on the Ag electrode, but it appears to be replaced by (CH₃)₄N⁺ as the potential becomes more negative. For the system Ag/MA, LiAsF₆ saturated with CO₂. Raman spectra of surface carbon have been recorded. A simple model to illustrate the surface processes in this nonaqueous system and implications for lithium cell technology are presented