In the case of Cs + we detect similar peak shifts of ester and amide bands by ~ -1 and ~ -6 cm -1 , respectively, when compared to K + . Modelling this situation using our DFT-predicted spectra requires a mixture of EB2:Cs + and EB:Cs + out (red and blue peaks/spectra, respectively, in Figure and 3A; see Figure ). By contrast, the pure species would predict negligible (-1 and -1 cm -1 ) or too large changes (+1 and -12 cm -1 ), respectively, with respect to spectra with K + , contrary to our SEIRA spectra. Figure shows spectra according to this interpretation (in a 1:2 mixture), again providing a satisfactory match to experimental spectra observed in Figure and.