Oscillations, Waves, and Interactions - GWDG

360 R. Pottel, J. Haller and U. Kaatze
range [50]. Ion complex formation of the subgroup metal ion Cu 2+ seems to be stabilized
by the Jahn-Teller effect. There are indications of an additional low-frequency
relaxation term in the spectra of CuCl2 solutions which has not been covered by the
measurements because transducer electrodes of the resonator cells were corroded by
the samples [50]. Likely this low-frequency term reflects the equilibrium between the
ion pairs and the outer-sphere complexes.
Also striking is the substantial increase in the amplitude A of calcium salt solutions
when chloride is substituted by nitrate (Table 3). We suppose stronger interactions
of (Ca 2+ )aq with the nitrate ion, due to its lone pair electrons and its delocalized π
orbital [104], than with the chloride ions with its spherical s 2 p 6 electron configuration.
An unexplained result is the considerably smaller difference in the amplitudes of
MgCl2 and Mg(NO3)2 solutions.
5 Conclusions
Multistep associations of ions have been among the first “immeasurable fast” reactions
[1] that have been effectively measured and described in detail [9,10]. Nevertheless
still today much interest is directed towards these reactions. Though the Eigen-
Tamm mechanism has been verified by broadband ultrasonic spectrometry of scandium
sulfate solutions [17] the existence of outer-outer sphere complexes in solutions
of 2:2 valent salts is still under discussion [32]. As early evaluations of experimental
spectra, without any assistance from computer facilities, used a slightly incorrect
high-frequency background term in the ultrasonic excess absorption spectra, a distribution
of relaxation times may have been simulated. More recent broadband spectra
of MnSO4 solutions (Fig. 9) reveal only two Debye-type relaxation terms within the
frequency range of measurements. These terms are assigned to the coupled equilibria
between completely dissociated ions and outer-sphere complexes and between
the latter and contact ion pairs. Halides of second subgroup metals, such as zinc
chloride, display a broad variety of ion complex structures in solution. Their spectra
have indeed been discussed in terms of a complex equilibrium between stoichiometrically
well-defined species [30] but have been also considered assuming fluctuating
ion clusters with intra- and intercluster correlations [31]. Small-amplitude ultrasonic
relaxation terms have been revealed for solutions of 2:1 valent electrolytes and have
been related to the equilibrium between the complex of encounter and the outersphere
complexes [34,35]. This equilibrium is of significance also for biochemistry as
it may interfere with other association mechanisms, like counter ion condensation on
polyelectrolytes, carbohydrate-cation interactions [107], as well as inclusion complex
formation [108].
Acknowledgments
We are indebted to the technicians of the institute for their continual support with
precision engineering components, electronic devices, computer facilities, and figure
layout.