Structure and motion in water. Analysis of vibrational and rotational dynamics of cyanide ion in aqueous solution from infrared and Raman bandshapes

Abstract

Infrared and Raman spectra of the CN stretching vibration of potassium cyanide in aqueous solutions have been recorded. Experimental infrared and anisotropic Raman bandshapes are found to be much more broad than the corresponding isotropic Raman profile and give evidence for a large rotational motion of the CN^– ion in aqueous solution. Changes in these bandshapes with respect to temperature confirm the existence of an orientational contribution.

Spectral profiles are analysed using the theory of Bratos et al., which allows the separation of vibrational and rotational components. From results obtained by changing the temperature, the pressure and the concentration, and by isotopic solvent exchange from H₂O/D₂O, one may conclude that, in the bandshapes, the vibrational part reflects a phase relaxation due to fluctuations in the anion environment. The theoretical result of the convolution of the isotropic Raman band by a rotational profile derived from a suitable model is compared with all the experimental results. It is shown that in aqueous solutions, the CN^– ion executes “instantaneous” rotations through large angles moving between sites of optimal interaction with the surrounding molecules of water. The rest time at the optimal sites is about 1 ps.

The activation energy of this reorientational motion is high (3 kcal mol^–1) and must be related to the energy needed to break a weak hydrogen bond between the anion and the water molecules.

The results are compared with previous thermodynamic and spectroscopic data which also show the “structure breaking” properties of the cyanide anion.