DFT calculations and normal mode analysis have been performed for a series of possible EBn:M z+ complexes with different stoichiometries and free EB in a polarizable continuum model (PCM) of n-pentadecane. The choice of PCM was to imitate the environment of the hydrophobic membrane in comparison to the SEIRA spectra. In principle one can imagine that EBn:M z+ complexes can preferably reside either in the membrane core or in the lipid head group region. However, the anisotropy of the dielectric constant of the head group region and the complexity of direct intermolecular interactions do not permit a straightforward and reliable prediction of such environments using DFT. In order to facilitate the comparison to the SEIRA spectra, simulated IR spectra are shown using Gaussian band shapes and FWHM values of 8 cm -1 and 16 cm -1 for v(COester) and v(COamide) modes, respectively, similar to the experimental IR bands. For each cation three different structural models have been simulated (Figure ): (a) a EB:M z+ -1:1 belted complex (yellow) and (b) a EB2:M z+ -2:1 sandwich complex (red), both shown schematically in Figure , as well as (c) a EB:M z+ -1:1 open-faced sandwich complex (blue), where one EB molecule is removed from a 2:1 complex. These structures will be referred to as EB:M z+ in, EB2:M z+ , and EB:M z+ out, respectively, in the following. The open-faced sandwich structure EB:M z+ out is a probable intermediate in the formation of EB2:M z+ complexes or in cation release from both EB2:M z+ and EB:M z+ in complexes and therefore may be observed in the SEIRA spectra. We are aware that in such an open-faced structure the cation will be ligated from the open face by either water molecules, anions or carbonyls or phosphate groups of lipids (or a combination of those) and consequently will reside at the membrane core/headgroup interface. If present, a mixture of these species will be detected using SEIRA, but it is not feasible to model the entire distribution of possible ligations using DFT for a comparison with experimental spectra. Thus we only simulated ligand-free EB:M z+ out structures in each case as a representative of these structures.