As obvious from SEIRA spectra with Li + , Mg 2+ , and Ca 2+ also here mixtures of species can be proposed, even though the assignment is aggravated by the broad spectral features. However, the observation of several bands in the v(COester) range and the feature at ~1560 -1550 cm -1 in Mg 2+ and Ca 2+ , which can be assigned to the ligating amide C=O groups, indicates the presence of either EB:M z+ in or EB:M z+ out structures and absence of EB2:M z+ (see summary in Figure ). This assertion is supported for Mg 2+ and Ca 2+ by the clear separation of the v(COamide) ~1560 -1550 cm -1 and the bands > 1600 cm -1 in SEIRA spectra, in between which the v(COamide) of EB2:M z+ would be observed according to DFT-based IR spectra (i.e. red peak at ~1610 cm -1 between yellow and blue feature in Figure ). For Li + , this assignment is solely based on the low probability of the highly asymmetric EB2:Li + in solution phase, where the singlyligating EB ring is expected to dissociate easily.