Determining the Structure of Oxalate Anion Using Infrared and Raman Spectroscopy Coupled with Gaussian Calculations

A laboratory project for the upper-division physical chemistry laboratory is described, and it combines IR and Raman spectroscopies with Gaussian electronic structure calculations to determine the structure of the oxalate anion in solid alkali oxalates and in aqueous solution. The oxalate anion has two limiting structures whose vibrational spectra have distinct differences: planar with D2h symmetry and nonplanar with D2d symmetry. In the former case, the IR and Raman spectra are complementary. Students measure the IR and Raman spectra of solid Na2C2O4, K2C2O4, and Cs2C2O4, and a nearly saturated aqueous solution of K2C2O4. They also carry out Gaussian calculations on the oxalate anion to predict the vibrational wavenumbers of both the planar and nonplanar structures and use the results to assign the spectra. By considering their results for the four samples, they decide on the oxalate structure in each and defend their choice in a report. The complexity of the project immerses the students in vibrational spectroscopy so that they learn the subject at a deeper level.