Raman study of vibrational and rotational dynamics of thiocyanate anion in aqueous solutions

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Abstract
The Raman spectra of the CN stretching mode of linear SCN- ions have been recorded in aqueous LiSCN, NaSCN and KSCN solutions at concentrations 1–10 M and at temperatures 30, 55 and 80°C. The vibrational and rotational correlation functions are calculated. The vibrational width of about 27–50 cm-1 in these thiocyanate solutions is attributed to the vibrational dephasing due to interactions between a SCN- ion and surrounding water molecules and cations. The observed vibrational correlation functions are analysed by the stochastic line shape theory of Kubo, in which homogeneous and inhomogeneous broadening are treated simultaneously, and both contributions to the spectra are extracted. The inhomogeneous contribution is found to increase with decreasing the temperature and in 10 M LiSCN solution at temperatures below 55°C the band is found to be broadened predominantly by inhomogeneous processes. Even in dilute solutions inhomogeneously slow modulation is found to exist besides the fast process which gives the dominant contribution to the observed vibrational width. Reorientation of the axis of a SCN- ion is found to be represented by the inertial motion of a single ion for short times (t ⪅ 0·15 ps, jump angle 17°) and by the rotational diffusion for longer times (t ⪆ 0·3 ps). A temperature study of the rotational correlation time τr allows us to estimate the activation energy of this reorientational motion to be about 5·6-10·2 kJ mol-1.