Complexation ultrasonic absorption

Ultrasonic absorption is used in the investigation of fast reactions in solution. If a system is at equilibrium and the equilibrium is disturbed in a very short time (of the order of 10"seconds) then it takes a finite time for the system to recover its equilibrium condition. This is called a relaxation process. When a system in solution is caused to relax using ultrasonics, the relaxation lime of the equilibrium can be related to the attenuation of the sound wave. Relaxation times of 10" to 10 seconds have been measured using this method and the rates of formation of many mono-, di-and tripositive metal complexes with a range of anions have been determined. 

In solution, [Co(terpy)2]2+ is also in a high-spin/low-spin equilibrium. Ultrasonic absorption measurements determined the spin equilibrium relaxation time in both water and MeOH solution to be less than 2 ns.249 Electron-donating functional groups such as methoxyl appended to the terpy ring result in a shift towards the high-spin form of the complex,250 as does replacement of one pyridyl ring with a pyrazole.251... 

Ultrasonic absorption played a major historical role in an understanding of the mechanisms of metal complex formation. It has also been used to study stereochemical change in metal... 

The separation of the two stages is easier to discern when the rates of the two processes are so different, but it can also be seen in the ultrasonic spectra of metal-sulfate systems (Sec. 3.4.4). Ultrasonic absorption peaks can be attributed to formation of outer-sphere complexes (at higher frequency, shorter t) and collapse of outer-sphere to inner-sphere complexes (at lower frequency). In addition to uv spectral and ultrasonic detection, polarimetry and nmr methods have also been used to monitor and measure the strength of the interaction. There are difficulties in assessing the value of ATq, the outer-sphere formation constant. The assemblage that registers as an ion pair by conductivity measurements may show a blank spectroscopically. The value of Aq at T" K may be estimated using theoretically deduced expres-... 

In the case of labile complexes, an equilibrium between inner and outer sphere complexes may exist, as has been concluded by Eigen and Tamm (36) from measurements of ultrasonic absorption. They have studied sulphate complexes of Be2+, Mg2+ and the divalent ions of the first series of transition elements from Mn2+ on. In all those cases, the outer sphere complexes have been found to predominate, the inner sphere complexes being only 10—20% of the total. The values of AH reported for these systems (22, 23, 35) would certainly suggest a higher proportion of inner sphere complexes, being closer to the values of AH found for complexes which are more or less firmly established as inner sphere than to those found for complexes established as outer sphere. This question merits further investigation. 

It is to be noted that irrespective of the weaknesses of equation (7.3), it has been extensively used in kinetic studies, especially in kinetics of fast complex formation reactions. The above equation was tested for its applicability to outer-sphere association reactions by ultrasonic absorption methods [8]. 

Another approach to determining the viscoelastic properties of dense microemulsions at high frequencies is to conduct ultrasonic absorption experiments. In such experiments it has been found that the percolation process is correlated to a shift of the ultrasonic dynamics from a single relaxation time to a distribution of relaxation times [121]. Other experiments showed an increase in the hypersonic velocity for samples at and beyond the percolation threshold. The complex longitudinal modulus deduced from such experiments is also correlated with the occurrence of the percolation phenomenon, which suggests that the velocity dispersion is clearly correlated with structural transformations [122]. 

The conformations of the alkali metal complex of A A and of its c-2 symmetric analog are identical, and this holds also for their solid and dissolved state. Complexation, apparently, renders the structure rigid. For the uncomplexed cyclopeptides, however, it turned out by measurement of CD, ultrasonic absorption [47a] and, mainly, NMR spectroscopy [47b] that there is a fast conformational equilibrium between up to four individual molecules depending on the polarity of the solvents. The stability of one conformer will be directed i.a. by the competing formation of hydrogen bonds intramolecularly vs H-donors or acceptors in the solvents. The important question as to the bio-active conformation has not been strictly answered so far. 

An ultrasonic absorption study of the kinetics for the formation of the second acetate complex of mercury(ii) in acidic aqueous solution, reaction (1), gives rate... 

Another ultrasonic absorption study of the rates of complexation-decom-plexation of 15-crown-5 and 18-crown-6 complexes for a series of bivalent cations in aqueous solution shows that non-spherically co-ordinated Hg + (similarly to the previously studied - Ag+) reacts faster with the crown ethers than spherically co-ordinated metal ions, which gives additional evidence for water loss as the rate-determining step. The more rigid crown ethers have to undergo considerable conformational strain to accommodate themselves to the linear geometry of silver(i) and mercury(ii), so the rigidity of the ligand is especially important for the rate of complexation with these metal ions. ... 

Crown Ethers, Cryptates, and Sepulchrates.— The results of an ultrasonic absorption study of the complexation of bivalent metal ions with 18-crown-6 and 15-crown-5 are consistent with a two-step mechanism,... 

Reactions in Water.—The ultrasonic absorption technique has been applied to silver(i) thiosulphate in solutions of which the predominant species is the ion [Ag(S203)2] . Both of the relaxation effects observed over the 15—230 MHz frequency range were attributed to the stepwise mutual approach of [AgS203] and [SaOal - with the elimination of co-ordinated water. At 25 °C the rate constant for this association reaction is 1.0 x 10 1 mol s, which is close both to the value calculated on the basis of the process being essentially diffusion controlled and to the literature value for the reaction of a second molecule of phen with the mono-complex [Ag(phen)]+. The authors favour an associative mechanism for these reactions of Ag+, with the bound ligand having essentially no effect on kr. 

The application of ultrasonic absorption to the kinetics of complex formation has been reviewed. This technique, which is particularly interesting from an historical standpoint, may frequently be used to determine the characteristic time of each elementary step in the overall formation of the complex. 

Our own chemical interests in complexation of inorganic ions by larger and larger ligands have prodded us to look for ultrasonic absorption techniques that would permit measurements below the low frequency end of the 15 MHz to MHz range ac-... 

The single relaxation observed in the ultrasonic absorption of copper(II) formate, acetate, and propionate has been used to evaluate a rate constant (20°C) for water substitution by carboxylate at Cu " of (3.4 1.2) X 10 s The enthalpy of activation is 10 1 kcalmoF for the formate, and it was confirmed that the ligand derived from the strongest acid forms the weakest copper complex. 

Zinc chloride and (under some conditions) zinc nitrate solutions in aqueous DMSO, on the other hand, yield " two ultrasonic absorption maxima with the chloride this is taken as evidence for an octahedral-tetrahedral coordination change accompanying the addition of the third bound Cr, while with the nitrate the high-frequency relaxation (observable only at low-water-mole fractions) is attributed to outer-sphere complex formation. The rate constants (25 C) for solvent loss with the nitrate (Xh20 = 0.59) and the chloride (Xhjo = 0.039 and 0.904) are, respectively, 2.2 X 10 , 4.1 X 10 , and 3.3 x lO s. ... 

Comparatively little work has been done on the kinetics of complex formation between the alkali metal ions and simple ligands in view of the high rate constants and low stability constants involved. Atkinson has recently studied the ultrasonic absorption of the five alkali metal sulfates in water in the frequency range 25-250 MHz, where he found only one relaxation for each salt. The results are analyzed in terms of the normal two-step mechanism (the fast formation of an outer-sphere complex followed by rapid conversion to the inner-sphere complex) in which the rates of the two steps approach each other as the concentration of the solution decreases. (The concentrations were in the range 0.3-1.0 mol dm". ) As expected, the reactions are nearly diffusion controlled the rate constants for inner-sphere complex formation at 0.5 mol dm and 25°C are 1.0 x 10 s for Li, Na, Rb, and Cs sulfates but 2.0 x 10 s for the potassium salt. 

The ultrasonic absorption of solutions of the EDTA complexes of Ca(II), Sr(II), Ba(II), and Co(II) has been ascribed to the configurational change of the complex in which the pentacoordinated structure is converted (with loss of the final water molecule from the inner coordination sphere of the metal) into the hexacoordinated structure. The values of A /(3.6 x 10, 9.2 x 10, 1.45 X 10 , and 9 x 10 s, respectively) are proportional to, but about one order of magnitude smaller than, the water-exchange rate constants at the unsubstituted... 

Thallium.— The chemistry of thallium(i) is usually compared to that of potassium(i) or silver(i). In studies of the complexation of a range of singly charged metal cations by aqueous 15-crown-5 (1,4,7,10,13-pentaoxacyclopentadecane) by ultrasonic absorption, the mechanism is described by equations (2) and (3), where CR and... 

The total solvation number can be calculated from the P value, provided the following two assumptions hold (i) the solvated (or complexed) ion is incompressible, (ii) the compressibility of the free solvent is the same as that of the pure solvent. On the other hand, the ultrasonic absorption of a system undeigoing chemical relaxation is given by... 

The rate of water exchange between the inner coordination sphere of rare earth ions and bulk water has been extensively studied by ultrasonic absorption and NMR spectroscopy. It is very fast and ranges between lO and 10 s (Cossy and Merbach 1988, Rizkalla and Choppin 1991). Ligand substitution processes and the kinetics of lanthanide complexation in water, especially with polyaminocarboxylates, have also been the subject of several studies (Lincoln 1986, Nash and Sullivan 1991). As for the determination of solvation numbers, investigations in anhydrous organic solvents are more sporadic. 

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