Infrared and Raman spectra of alkaline-earth and copper(II) acetates in aqueous solutions

Abstract : Attenuated total reflectance infrared and Raman spectra of dilute aqueous solutions (0.05 to 0.6 M) with alkaline-earth or copper(II) cations and acetate anions are studied at room temperature. Four carboxylate structures are observed in the solutions studied: protonated, free, pseudobridging between a water proton and a copper ion and, probably, bidentate. These species are related to partially resolved components of the ν(C–C) modes at 892, 928, 938 and 948 cm−1, respectively, in Raman spectra. A quantitative comparison of Raman spectra with distribution diagrams calculated from the complexation constants given in literature allows the assignment of the component at 938 cm−1 to both hydrated complexes CuCH3COO+ and Cu(CH3COO)2. In the complex Cu(CH3COO)3−, some carboxylate anion(s) contribute to the scattering at 948 cm−1 and, probably, the other(s) at 938 cm−1. The carboxylate structures related to these last ν(C–C) wavenumbers have been assessed by using also spectra of the carboxylate stretching modes. With full widths at half height close to 12 cm−1, the ν(C–C) bands appear as better probes of the various carboxylate species than the carboxylate stretching modes which may overlap with other vibrations and whose widths are 3 to 5 times larger. The possibility to distinguish the spectra of a contact-ion-pair and a solvent separated ion-pair involving acetate and sodium or alkaline-earth ions is discussed. No distinction has been observed, even in the ν(C–C) range of Raman spectra, because the contact-ion-pairs are a small minority with the experimental conditions used.
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F. Quilès, A. Burneau. Infrared and Raman spectra of alkaline-earth and copper(II) acetates in aqueous solutions. Vibrational Spectroscopy, Elsevier, 1998, 16 (2), pp.105 - 117. ⟨10.1016/S0924-2031(98)00004-6⟩. ⟨hal-01935498⟩

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