Raman band shape analysis of a low temperature molten salt

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Abstract
The salt tetra(n-butyl)ammonium croconate, [(n- C 4 H 9 ) 4 N ] 2 C 5 O 5 ⋅4 H 2 O , (TBCR), is a very viscous glassforming liquid which undergoes a glass transition at room-temperature. Raman band shape analysis of the totally symmetric ring breathing mode of the croconate dianion, C 5 O 5 2− , was performed by Fourier analysis. The vibrational time correlation functions obtained from the isotropic Raman spectra were modelled with well-known models for vibrational dephasing. The time correlation functions of pure TBCR and of TBCR in acetonitrile solutions were compared with previous results for the simple salt Li 2 C 5 O 5 in aqueous solution. It has been found remarkable changes of the dynamic parameters characterizing the vibrational dephasing of C 5 O 5 2− in these different environments. Discontinuous temperature dependence of the dephasing parameters was observed at the glass transition temperature of pure TBCR. In glassy TBCR, however, common models for vibrational dephasing are not strictly valid because the Raman bands display clear asymmetric shapes. The experimental data in glassy TBCR were also reproduced with a model that considers the second and the third order terms in the cumulant expansion of the vibrational correlation function.