Ultrasonic relaxation complexes

This report has been written in order to demonstrate the nature of spin-state transitions and to review the studies of dynamical properties of spin transition compounds, both in solution and in the solid state. Spin-state transitions are usually rapid and thus relaxation methods for the microsecond and nanosecond range have been applied. The first application of relaxation techniques to the spin equilibrium of an iron(II) complex involved Raman laser temperature-jump measurements in 1973 [28]. The more accurate ultrasonic relaxation method was first applied in 1978 [29]. These studies dealt exclusively with the spin-state dynamics in solution and were recently reviewed by Beattie [30]. A recent addition to the study of spin-state transitions both in solution and the... 

The most significant results obtained for complexes of iron(II) are collected in Table 3. The data derive from laser Raman temperature-jump measurements, ultrasonic relaxation, and the application of the photoperturbation technique. Where the results of two or three methods are available, a gratifying agreement is found. The rate constants span the narrow range between 4 x 10 and 2 X 10 s which shows that the spin-state interconversion process for iron(II) complexes is less rapid than for complexes of iron(III) and cobalt(II). 

Fe(acac2trien)]N03. For the iron(III) complex of the hexadentate Schiff-base ligand acac2trien, the barrier heights have been determined from ultrasonic relaxation [94] as AG[h = 6-28 kcalmoD = 2196 cm and AGJil = 5.85 kcalmol= 2046 cm The difference of zero-point energies has been obtained from equilibrium studies as AG° = 0.43 kcal mol =... 

The observation of a single set of resonances in the NMR spectra of [Fe(HB(pz)3)2], spectra that are clearly obtained for a mixture of the high-spin and low-spin forms of the complex, indicates that the equilibrium between the two states is rapid on the NMR time scale [27]. Subsequent solution studies by Beattie et al. [52, 53] using both a laser temperature-jump technique and an ultrasonic relaxation technique have established that the spin-state lifetime for [Fe(HB(pz)3)2] is 3.2xl0 8 s. These studies also established... 

Studies on the dynamics of complexation for guests with cyclodextrins have been carried out using ultrasonic relaxation,40 151 168 temperature jump experiments,57 169 183 stopped-flow,170,178,184 197 flash photolysis,57 198 202 NMR,203 205 fluorescence correlation spectroscopy,65 phosphorescence measurements,56,206 and fluorescence methods.45,207 In contrast to the studies with DNA described above, there are only a few examples in which different techniques were employed to study the binding dynamics of the same guest with CDs. This probably reflects that the choice of technique was based on the properties of the guests. The examples below are grouped either by a type of guest or under the description of a technique. 

Table 11 Equilibrium constants and association and dissociation rate constants for alcohol/ CD complexes determined by ultrasonic relaxation at 25°C...

Resonator ultrasonic relaxation method, 32 18 Respiratory chains, 45 351-354 aerobic growth, 45 354-357 anaerobic growth, 45 357-359 autotrophic growth, 45 359-362 complexes, proteins, 38 240-241 membrane-bound Fe—S enzymes, 38 302-303... 

The pressure dependence of the NMR spectrum of a nickel(II) complex which undergoes a coordination-spin equilibrium has been used to obtain the volume difference between the planar and octahedral isomers (118). In this case both the temperature and pressure dependence of the NMR spectra were analyzed simultaneously to yield five parameters, AH0, AS0, A V°, and the chemical shifts of the two isomers. Subsequent determinations from the electronic spectra and ultrasonics relaxation are in good agreement with the NMR result (13). 

The dynamics of an octahedral spin equilibrium in solution was first reported in 1973 for an iron(II) complex with the Raman laser temperature-jump technique (14). A relaxation time of 32 10 nsec was observed. Subsequently, further studies have been reported with the use of this technique, with ultrasonic relaxation, and with photoperturbation. Selected results are presented in Table III. 

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. 

The spin-equilibrium dynamics of iron(III) complexes in solution have been examined with the techniques of Raman laser temperature-jump, ultrasonic relaxation, and photoperturbation. The complexes investigated, the relaxation times observed, and one of the derived rate constants are presented in Table IV. Many of the relaxation times are quite short, and some of the original temperature-jump results (45) were found to be inconsistent with more accurate ultrasonic experiments (20) and later photoperturbation experiments (102). It has not been possible to repeat some of these laser temperature-jump observations. Instead, the expected absorbance changes and isosbestic points were found to occur within the heating rise time of the laser pulse, consistent with the ultrasonic and photoperturbation experiments (20). Consequently, none of the original Raman laser temperature-jump results is included in Table IV. 

The ultrasonic relaxation kinetic technique measures faster reactions than the NMR method and thus is able to detect more steps in the crown ether complexation sequence. This is exemplified by a study of the complexation of LiC104 by 18C6 in propylene carbonate in which two relaxations are detected which are attributed to the operation of an Eigen-Winkler mechanism for Li exchange as in Eq. (5). 

These interconversions are quite rapid and have been studied by laser T-jump" and ultrasonic relaxation. Kinetic studies suggest that several nickel(II)-salicylaldimine complexes favor ligand substitution through the square-planar isomer. 

Transition Metals M (except Ni +).— Ultrasonic relaxation data on Cu(C104)2 in methanol at 25 °C are consistent with an /d mechanism. Representing the formation of the mono-complex by equation (1) (where S = MeOH),... 

There have been further kinetic investigations of the complexes formed between the alkali and alkaline earth cations and various neutral polycyclics. Eyring and coworkers have studied - the reaction of 18-crown-6 (3) with the aqueous cations Li, Na+, K+, Rb+, Cs+, T1+, Ag+, NH4+, and Ca + by means of the ultrasonic relaxation method. The data are analysed in terms of the two-step model proposed by Chock for the complexation of univalent ions by dibenzo-30-crown-10 (4) in methanol... 

Ultrasonic relaxation absorption has been observed " in aqueous solutions of Zn(II) and Cd(II) edta complexes. The absorption is ascribed to a rapid structural change of the complex itself involving penta- and hexacoordinated forms of the ligand, and in both cases the rate constant (5 — 6) is close to the water substitution rate constant of the metal ion. NMR spectroscopy has been used " to demonstrate that a similar dynamic equilibrium exists in the edta complexes of Co ", Ni ", and Cu ", although in the case of the Mn " complex the ligand is present only in the pentacoor-dinate form. In the reaction of Mg ", Ca ", Sr ", Ba ", Ni ", Cu ", or Zn ... 

The dynamics of the square-planar - octahedral isomerization of a nickel(II) complex with N-(2-piperidinoethyl)salicylaldiminato ligands has been studied by ultrasonic relaxation methods- O This rearrangement can be represented as follows. Scheme 11.8 ... 

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... 

A number of observations indicate interconversion rates for some iron(III) complexes too fast to measure with existing techniques. Relaxation times less than the 30-nsec limit set by the heating rise time of the laser temperature-jump technique were observed for [Fe(benzac2trien)]+, [Fe(Salmeen)2]+, and [Fe(Me2dtc)3] (45, 128). From ultrasonic observations a limit of less than 1 nsec was placed on the relaxation time for the first of these compounds,... 

The kinetics and dynamics of crvptate formation (75-80) have been studied by various relaxation techniques (70-75) (for example, using temperature-jump and ultrasonic methods) and stopped-flow spectrophotometry (82), as well as by variable-temperature multinuclear NMR methods (59, 61, 62). The dynamics of cryptate formation are best interpreted in terms of a simple complexation-decomplexation exchange mechanism, and some representative data have been listed in Table III (16). The high stability of cryptate complexes (see Section III,D) may be directly related to their slow rates of decomplexation. Indeed the stability sequence of cryptates follows the trend in rates of decomplexation, and the enhanced stability of the dipositive cryptates may be related to their slowness of decomplexation when compared to the alkali metal complexes (80). The rate of decomplexation of Li" from [2.2.1] in pyridine was found to be 104 times faster than from [2.1.1], because of the looser fit of Li in [2.2.1] and the greater flexibility of this cryptand (81). At low pH, cation dissociation apparently... 

Three groups of phenomena affect the frequency-dependence of ultrasonic wave propagation classical processes, relaxation, and scattering, of which scattering is likely to dominate in foodstuffs due to their particulate nature. The two classical thermal processes are radiation and conduction of heat away from regions of the material, which are locally compressed due to the passage of a wave they can lead to attenuation but the effect is negligible in liquid materials (Herzfield and Litovitz, 1959 Bhatia, 1967). The third classical process is due to shear and bulk viscosity effects. Attenuation in water approximates to a dependence on the square of the frequency and because of this it is common to express the attenuation in more complex liquids as a()/o or a(f)jf2 in order to detect, or differentiate from, water-like properties. 

The technique of ultrasonic impedometry was developed by Mason and McSkimmin of Bell Telephone laboratories for measuring the complex mechanical shear modulus of polymer liquids (11, 12). They described three relaxation modes for the systems—configurational elasticity, long... 

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