Comparing the vibrational frequencies within the monovalent series for the open-faced EB:M x+ out structure (Figure blue spectra and box), which in all cases let to a C3-symmetric conformations, provides insight into the cation-dependent trends within one type of complex. Accordingly, the v(COester) frequency is predicted to vary according to the series Ca 2+ > Li + > Mg 2+ > Na + > K + > Cs + by about 10 cm -1 . These predictions are overall in line with expectations from the vibrational Stark effect when taking into account the out-facing orientation of the ester carbonyls, and the changing charge density of the ions. For the amide stretch we note the inverted trend of Cs + ≥ K + ≥ Na + ≥ Li + >> Mg 2+ ≈ Ca 2+ over almost 100 cm , where one can expect that additional electronic polarization of the amides play a considerable role in addition to the vibrational Stark effect. Importantly, the simulated spectra show that within similar types of complexes (see EB2:M z+ -red spectra; and EB:M z+ out -blue spectra) peak positions with K + and Cs + differ by only ±1 cm -1 and are therefore not easily distinguished; this is also the case comparing Mg 2+ and Ca 2+ . Instead, in the EB:M z+ out (blue spectrum) with Na + the ester and amide C=O band are shifted by +1 cm -1 and -4 cm -1 , respectively, relative to K + ; with Li + this shift is more pronounced to +3 and -16 cm -1 , and with Mg 2+ /Ca 2+ to +8 and -82 cm -1 . Finally, non-ligating EB gives a very distinctive spectrum, which is easily distinguished from most EB:ion complexes by closely positioned ester and amide bands at 1715/1685 cm -1 . This information gives valuable guidelines for the analysis of the recorded spectra.