Coordination and structure of Ca(II)-acetate complexes in aqueous solution studied by a combination of Raman and XAFS spectroscopies

Structure and complexation mechanism of aqueous Zn(II)-acetate complex studied by XAFS and Raman spectroscopies

“…The ∆ s-a value for the ionic state (deprotonated acid) is employed as a reference for determining if a complex is bidentate ( ∆ s-a lower than reference) or mono-dentate ( ∆ s-a significantly larger than reference). This model has been in good agreement with experimental results for Ca [ 9 , 10 ] or Mg [ 11 ] complexes. In the case of Zn, it has been extensively debated about the mono-dentate or bidentate interaction with the acetate ligand [ 12 ].…”

“…In a previous paper, we investigated the complexation of Ca(II)-acetate, and the main bands in the vibrational spectra were identified for this complex based on the literature [ 10 , 23 – 25 ]. To study the complexation of the cation, special attention was paid to carbonyl (C=O) and COO symmetrical and asymmetrical stretching vibrational modes (ν s COO and ν a COO, respectively).…”

“…The Raman bands for the protonated acetate (Ac-H at 878 cm −1 ) and the C–C symmetric stretching (ν s CC at 915 cm −1 ) are also sensitive to dissociation and complexation. The ν s CC band is strongly coupled to the vibrational modes of the carboxyl and is employed to determine the complexated/non-complexated acetate in solution [ 10 ]. This band presents a band splitting for large cations 10 but is not resolved in the case of Zn.…”

“…We have preliminary compared the O-C-O out-of-plane rocking modes of the acetate upon dissociation with the corresponding to the complexation with Zn cations. The data presented as supplementary information (Figure S4), can be compared to the spectra measured in equivalent Ca(II)-acetate solutions [ 10 ], showing noticeable differences. For instance, the rocking band in the Zn(II)-acetate shifts clearly to lower frequencies as the dissociated species ratio increases (about 30 cm −1 ).…”

XPS as a probe for the bonding nature in metal acetates

“…Much effort in the spectroscopic characterization of metal carboxylates has focused on the distinction of carboxylate coordination modes, especially in cases where no structural information is available or when the experimental conditions are not suitable for making use of X-ray diffraction. For this endeavor, NMR, 16,17 Raman, 18,19 and IR 20 techniques have been employed with success, although they present several limitations, with perhaps their poor surface resolution being the most relevant. In this regard, X-ray photoelectron spectroscopy (XPS) has been established as a powerful tool for the analysis of material surfaces and the study of interfacial processes 21 due to its high surface sensitivity and reliable information about chemical bonding, especially when the photoelectron and Auger lines for an atom are combined to calculate the so-called Auger parameter ( α ′).…”