On the Aqueous Vibrational Spectra of Alkali Metal Oxalates

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Abstract
Fourier transform infrared attenuated total reflectance and Fourier transform Raman spectra of the series of aqueous alkali metal oxalates—lithium oxalate (Li2C2O4), sodium oxalate (Na2C2O4), potassium oxalate (K2C2O4), rubidium oxalate (Rb2C2O4), and cesium oxalate (Cs2C2O4)—are presented for the first time. Fourier transform Raman spectra of the solid oxalates are also presented for the first time. The solid and aqueous oxalate ions are assumed to possess D2 h and D2 d symmetry, respectively, and the assignment of fundamental vibrational modes is made accordingly. The effect of increasing alkali metal ion concentration on the aqueous spectra of these oxalates is also reported. Spectral changes are explained in terms of the ability of the respective alkali metal cations to interact with the oxalate anion in solution. It is proposed that these differences arise because the alkali metal cations associate with the aqueous oxalate anions to varying degrees, depending upon the nature of the cation. The extent of the binding is found to decrease down the alkali metal series (Li > Na > K > Rb > Cs). Similar results obtained for the tetramethylammonium cation [(CH3)4N+] suggest that it is a softer cation than cesium.