Raman bandshape analysis of oxocarbon ions in aqueous solutions

Abstract

Fourier analysis of Raman bandshapes was undertaken for some modes of the lithium salts of squarate (C 4O 4 2−) and croconate (C 5O 5 2−) ions in saturated aqueous solution. Time correlation functions obtained from the isotropic and anisotropic Raman components of the totally symmetric modes of ring breathing, and the ones obtained from the anisotropic components of the non-totally symmetric modes of CC stretching and ring bending, have similar decay rates for both C 4O 4 2− and C 5O 5 2−, indicating that reorientation is very hindered in these systems for both spinning and tumbling of the main symmetry axis. The experimental vibrational correlation functions were fitted by the Kubo's equation, and are in the intermediate modulation regime, although faster than the ones for other typical ions reported in literature. The small time correlation of vibrational frequency fluctuations (≈ 0.1 ps) together with slow reorientation imply the picture of the ions performing librational motions inside temporary cages formed by water molecules. From the modeling of the vibrational and reorientational correlation functions, the estimated average librational frequency is ca. 80 cm −1. Known models which separate homogeneous and inhomogeneous contributions give no better fitting of the vibrational correlation functions. The homogeneous nature of the vibrational dephasing process is also pointed by the almost perfect Lorentzian shape of the bands and by the calculation of the isotropic components by a memory function approach truncated in second order. Similar results were found after a 1:10 dilution of the saturated solutions, or upon increasing the temperature to 50°C, or yet by using of the sodium salts.