INFRARED AND RAMAN-SPECTRA OF ALKALINE-EARTH AND COPPER(II) ACETATES IN AQUEOUS-SOLUTIONS

Attenuated total reflectance infrared and Raman spectra of dilute aque ous solutions (0.05 to 0.6 M) with alkaline-earth or copper(II) cation s and acetate anions are studied at room temperature. Four carboxylate structures are observed in the solutions studied: protonated, free, p seudobridging between a water proton and a copper ion and, probably, b identate. These species are related to partially resolved components o f the nu(C-C) modes at 892, 928, 938 and 948 cm(-1), respectively, in Raman spectra. A quantitative comparison of Raman spectra with distrib ution diagrams calculated from the complexation constants given in lit erature allows the assignment of the component at 938 cm(-1) to both h ydrated 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 str uctures related to these last nu(C-C) wavenumbers have been assessed b y using also spectra of the carboxylate stretching modes. With full wi dths at half height close to 12 cm(-1), the nu(C-C) bands appear as be tter probes of the various carboxylate species than the carboxylate st retching modes which may overlap with other vibrations and whose width s are 3 to 5 times larger. The possibility to distinguish the spectra of a contact-ion-pair and a solvent separated ion-pair involving aceta te and sodium or alkaline-earth ions is discussed. No distinction has been observed, even in the nu(C-C) range of Raman spectra, because the contact-ion-pairs are a small minority with the experimental conditio ns used. (C) 1998 Elsevier Science B.V. All rights reserved.