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

• Published in: Journal of molecular structure 2018-06, Vol.1161, p.512-518
• Main Authors: Muñoz Noval, Álvaro, Nishio, Daisuke, Kuruma, Takuya, Hayakawa, Shinjiro
• Format: Article
• Language: English
• Subjects: Ca K-Edge; Calcium acetate; Chelating agents; Complex structure in solution; Raman; XAFS
• ISSN: 0022-2860; 1872-8014
• DOI: 10.1016/j.molstruc.2018.02.075

The determination of the structure of Ca(II)-acetate in aqueous solution has been addressed by combining Raman and X-ray absorption fine structure spectroscopies. The pH-dependent speciation of the acetate/Ca(II) system has been studied observing modifications in specific Raman bands of the carboxyl group. The current results evidence the Ca(II)-acetate above acetate pKa forms a bidentate complex and presents a coordination 6, in which the Ca-O shell radius decrease of about 0.1 Å with respect the hydrated Ca2+ with coordination 8. The experimental results show the OCO angle of the carboxyl in the complex is close to 124°, being the OCaO angle about 60°. •A combination of Raman and XAFS have been applied to determine the structure of Ca(II)-acetate in aqueous solution.•The analysis of the νC-C and Ac-H bands instead of asymmetric and symmetric stretching COO− bands allows to avoid signal from dimerization.•The hydrated Ca(II) ion acquires a coordination 8, once the Ca(II) complexates with the acetate group, the ion adopts a coordination 6.•The carboxyl angle in the complex (θOCO) has been obtained by EXAFS for the Ca(II)-acetate, is about 124°, in good agreement with the values obtained by Nara's equation.