Ultrasonic relaxation interest

Consideration of the thermodynamics of a representative reaction coordinate reveals a number of interesting aspects of the equilibrium (Fig. 5). Because the complex is in spin equilibrium, AG° x 0. Only complexes which fulfill this condition can be studied by the Raman laser temperature-jump or ultrasonic relaxation methods, because these methods require perturbation of an equilibrium with appreciable concentrations of both species present. The photoperturbation technique does not suffer from this limitation and can be used to examine complexes with a larger driving force, i.e., AG° 0. In such cases, however, AG° is difficult to measure and will generally be unknown. 

It appears interesting to note that quite recently Zana224 also claimed to have observed (with DAP in cyclohexane applying an ultrasonic relaxation technique) a decrease of the average relaxation frequency with increasing DAP concentration. 

The experimental techniques used to obtain the properties necessary to derive the kinetic constants of interest from the ultrasonic relaxation times have been previously described in detail [2,3]. Briefly, the degree of micelle ionization (P) and the binding constant (Ka) of an alcohol to mixed micelles were obtained from specific conductivity measurements as a function of surfactant concentration at various fixed alcohol compositions. The binding constant was determined from the slopes of the curves above the cmc, as proposed by Abu-Hamdiyyah et al... 

When C is smaller than 0.1 M the IR-data indicate that K=78 10%M. Consequently it can be concluded that the ultrasonic relaxation spectrum is less sensitive to concentration variations than the IR spectrum and therefore may be described by the simpler approximation over a larger concentration range. It seems interesting to mention that the two state type of approximation may give the clue to the understanding of why very complicated association systems very often cause surprisingly simple relaxation spectra. 

ULTRASONIC RELAXATION STUDIES OF THE KINETICS OF STACKING, CONFORMATIONAL TRANSITIONS AND PROTON TRANSFERS IN AQUEOUS SOLUTIONS OF MOLECULES AND MACROMOLECULES OF BIOLOGICAL INTEREST... 

The ultrasonic relaxation observed in CTAB/Pentanol/water-systems has previously been interpreted in terms of a perturbation of the equilibrium between alcohol molecules in the water phase and micelle phase respectively. Generally speaking the sound wave detects the alcohol molecules jumping in and out of the CTAB-micel-les. The interesting question is whether or not this process is affected by additives that make the system viscoelastic. It is important to notice that the ultrasonic absorption data at different frequencies for CTAB in pure water are pretty close to those obtained from CTAB made viscoelastic by adding Sodium Salicylate. Thus the ultrasonic wave hardly detects the change that causes the viscoelasticity. 

In Wat . —There has been renewed interest recently in ultrasonic relaxation in aqueous salt solutions with the appearance of several papers on bivalent and tervalent sulphates. Jackopin and Yeager have presented some new data on manganese(n) sulphate and have critically re-examined the question of the number of relaxation peaks observed with this species. 

By the same technique of resonance and pulsed ultrasonic perturbation Beattie et al.176) have also studied the spin crossover dynamics in aqueous solutions of [Fe(paptH)2]Cl2- Relaxation times of ca. 40 ns were measured at 25 °C, again independent of concentration and thus presumably arising from a unimolecular spin isomerization process. Some of the thermodynamic parameters of particular interest are here AH0 = 3.91 kcal mol-1, AS° = 14.8 eu, AH = 7.63 kcal mol-1, AH = 3.72 kcal mol-1,, , = 10-2S. 

An interesting application of ultrasonic measurements was to aqueous solutions of certain metal salts. The sound-absorption characteristics of several 2 2 electrolytes of the type MX have been measured, and found to have two maxima (cf Fig. 12) from which two relaxation times can be determined ". It was not obvious with which relaxation processes these times should be associated, especially since the (diffusion-controlled) rate coefficients for ion-pair formation (which is known to occur at the concentrations used, 10 -10 M) give relaxation times too short to be observed. It seemed, then, that when the two ions and had... 

Several excellent reviews in related and parallel areas have appeared recently. " Though much early work in the field concentrated on vibration-translation/rotation relaxation phenomena using ultrasonic methods, a remarkable resurgence of interest in the area occurred with the advent of infrared lasers and their application to laser-induced infrared... 

The observed dipole relaxation in benzene is therefore ascribed to this transient species. It is of interest that the ultrasonic and dielectric relaxation features in this instance reflect the same relaxation process this is not always the case. 

Two problems of continuing interest in lanthanide chemistry are the mechanisms of complex formation in aqueous and other media and the question of whether the solvation-number change in the middle of the series is a function of the cation alone. Previous relaxation measurements have suggested that the spectroscopic and kinetic data for Er(N03)3 could only be explained by invoking a solvation-number change in mixed solvents. A preliminary communication on the ultrasonic absorbance of aqueous-... 

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