Dynamic structures of aqueous oxalate and the effects of counterions seen by 2D IR

Two dimensional vibrational echo spectra of oxalate in the carboxylate asymmetric stretch region in D(2)O show two transitions having anomalously slow spectral diffusion and a third transition having relaxation properties typical of the free carboxylate ion. Quantitative analysis of the frequency shifts of the carboxylate asymmetric stretch modes caused by a singly charged cation in the oxalate hydration shell supports that ion pairs can be responsible for these new transitions. Experimental evidence and DFT calculations are consistent with oxalate forming a mixture of "side-on" and "end on" contact ion pairs wherein the carboxylate groups are protected from mobile heavy water molecules.