ELSEVIER Chemical Physics 217 (1997) 71-81 Chemical Physics Raman bandshape analysis of oxocarbon ions in aqueous solutions l Mauro C.C. Ribeiro a,* ,2 Luiz F.C. de Oliveira b Paulo S. Santos a a Laborat6rio de Espectroscopia Molecular, lnstituto de Qufmica, Universidade de S~o Paulo, CP 26077, CEP 05599-970 SEtoPaulo, Brazil b Departamento de Qu[mica, Instituto de Ci~ncias Exatas, Universidade Federal de Juiz de Fora, CEP 36036-330Juiz de Fora, MG, Brazil Received 9 September 1996 Abstract Fourier analysis of Raman bandshapes was undertaken for some modes of the lithium salts of squarate (C4O2-) and croconate (C50~-) 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 C402- and C502-, 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-~. 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. Keywords: Squarate ion; Croconate ion; Raman bandshape 1. Introduction Raman spectroscopy, with the possibility of ob- taining polarized and depolarized spectra, became * Corresponding author. f This paper is dedicated to the 70 th birthday of Prof. Oswaldo Sala. 2 Present address: Physical and Theoretical Chemistry Labora- tory, Oxford University, South Parks Rd., Oxford, OXI 3QZ, UK. one of the main tools for unveiling the vibrational and reorientational relaxation dynamics of molecular species as neat liquids or solutions [1-3]. Particu- larly for aqueous solutions, the advantages of Raman spectroscopy comparatively to infrared spectroscopy are rather obvious. In addition, from the polarized and depolarized spectra, it is possible to obtain, individually, the vibrational and reorientational con- tributions, provided that a standard assumption on their separability is considered [4,5]. It is generally 0301-0104/97/$17.00 Copyright © 1997 Elsevier Science B.V. All rights reserved. PII S0301-0 104(97)00032-3