Assignment for the Infrared Spectrum of Solid Sodium Propionate from Low-Temperature Measurements in Combination with 13C Isotopic Shifts

The infrared spectra of CH3CH2COONa and its l3C-labeled modifications (1-13C, 2-13C, and 3-13C) suspended in KBr disks were measured in the region 4000–200 cm−1 at room temperature and liquid nitrogen temperature. Overlapping complex band contours appeared in some regions of the room temperature spectrum, most notably in the region 1500–1350 cm−1, where 5 fundamentals having contributions from the methyl deformation, methylene bending, and carboxylate stretching modes should occur. In contrast to this, excellent resolution was reached at the low temperature, from which all 22 fundamentals expected in the whole spectral region investigated were detected. A complete assignment of the fundamentals is proposed mainly on the basis of the characteristic isotopic shifts of the three 13C substituted sodium propionate species. A fair number of the fundamentals were found to feature coupled modes having contributions from several group vibrations.