These ion-dependent and diverse structures have consequences on the IR spectra computed via normal mode analysis (Figure left). Overall, simulated IR spectra support the previous assignment of the high-frequency and low-frequency bands in the 1750 -1600 cm -1 region to the ester (~1738 cm -1 ) and amide carbonyl stretches (1660 -1640 cm -1 ), respectively, in presence of Na + , K + and Cs + . For C3-symmetric structures, these bands are dominated by the symmetric mode and a nonappreciable contribution of the anti-symmetric modes occurring as redshifted shoulders (by ca. 6 cm -1 ). Non-coordinating ester carbonyls are predicted to absorb in the range around 1738 cm -1 for EB2:M z+ and EB:M z+ out open-faced complexes. In EB:M z+ in complexes the ester groups contribute to complexation and are thus shifted to ca. 1695 cm -1 for Na + and Li + , which can be used as a specific marker for the EB:M z+ in complex with monovalent cations. The coordinating v(COamide) is located at 1660 -1625 cm -1 . In cases where the amide does not coordinate to the metal, it is predicted to absorb closer to 1680 cm -1 . With bivalent cations the ligating ester and amide groups are significantly redshifted to roughly 1640 and 1580 cm -1 , respectively, as a consequence of the high charge density of the metal ions.