8575
J . Phys. Chem. 1991, 95, 8575-8580
An I R Study of the Hydration of Ci0,-, NO3-, I-, B r , Cr, and SO-: Solution
Anlons in Aqueous
Pehr- Ake Bergstrom, Jan Lindgren,* Institute of Chemistry, University of Uppsala, Box 531, S-751 21 Vppsala, Sweden
and Olof Kristiansson Department of Chemistry, Swedish University of Agricultural Sciences, Box 7015, S - 750 07 Vppsala, Sweden (Received: March 21, 1991)
The hydration of Clod-, NO3-, I-, Br-, C1-, and S O : - anions has been studied in aqueous solution. A double-difference IR spectroscopic method has been applied, whereby OD spectra of isotopically isolated HDO molecules in the hydration spheres of the anions are obtained after the contributions from H 2 0 and HDO in the bulk are removed. Each anion is shown to interact with its neighboring water molecules, which gives rise to OD vibration bands with characteristic peak frequencies and bandwidths. The bandwidth is found to increase smoothly with increasing band position. The OD stretching intensity for hydrating water (HDO) molecules has been found to decrease linearly with band position. Band positions have been converted to anion-water distances by means of correlation curves. These have been compared to available X-ray and neutron diffraction data on solutions and solid hydrates.
Introduction
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The water molecules that hydrogen bond to anions in aqueous solution exchange quite rapidly: 10" s-' for halide ions.'2 This implies that our choice of methods for studying anion hydration is limited. For example, the N M R technique cannot be used as it responds to effects on a much longer time scale. Several X-ray and neutron diffraction results of halide ion hydration in solution have been r e p ~ r t e d . Diffraction ~ methods applied to aqueous ionic solutions result in complex radial distribution functions, consisting of several overlapping paircorrelation functions. This has brought about the use of isostructural substitution, e.g., taking advantage of the different neutron scattering lengths of atomic isotopes in calculating first- and second-order difference scattering function^.^ The applicability of neutron diffraction methods relies heavily on the accessibility of suitable atomic isotopes, and a characteristic anion hydration shell has so far only been established for the chloride i ~ n . ~ J An alternative approach, applicable to polyatomic anions, is the use of IR or Raman spectroscopy to study shifts in the intramolecular stretching vibrations of the anions. Such shifts have been correlated to hydration phenomena as reported, for example, for the nitrate ion.6 Even though the water molecules neighboring the anions exchange rapidly, this is nevertheless relatively slow compared to the intramolecular stretching frequency for the water molecule: lof4s-I. Thus, vibrational spectroscopy offers the possibility of studying anion hydration through the effect these anions have on the stretching vibrations of their neighboring water molecules on a time scale which allows the observation of relatively shortlived species. In the present study, the hydration of C104-,NO
