Kinetic parameters group complexes

Kinetic parameters for aquation at corresponding Cr(III) and Co(III) complexes have been compared for a series of complexes cis-[ML4XY]"+, where L4 = (NH3)4 or (en)2, X = Cl- or H20, and Y=an uncharged leaving group (DMSO, DMF, or DMAC). The uniformly negative activation volumes (AV between —2 and —11 cm3 mol-1) for the chromium complexes contrast with uniformly positive activation volumes (A V between +3 and +12 cm3 mol-1) for the cobalt complexes - AV values provide a more clear-cut contrast than AS values here (22). 

A variety of small molecules can be inserted between the two metals in these complexes. The reaction with CH2N2 in CHjClj to insert — CH2- is second order and kinetic parameters for different X s suggest a rate-limiting transfer of an electron pair from the Pt—Pt bond to the methylene group of CH2N2 ... 

Several different models were proposed for the slow NOx storage process, while only few details and approximate models are available for the highly transient NOx reduction within the rich phase, lasting only several seconds. The models can be divided into two groups, depending on whether the internal diffusion in the particles of the NOx storage material is considered explicitly, or this effect is included implicitly into the evaluated kinetic parameters. The models can be further differentiated by the level of complexity for the reaction kinetics description, i.e., either (simplified) microkinetic scheme or the global kinetics. 

In another investigation,425 the exchange between [Ce(edta)aq] and hydrated Pb2+, Ni2+ or Co2+ ions again show reaction by dissociation of protonated [Ce(Hedta)aq] as well as by the direct attack of metal ions on [Ce(edta)aq] or [Ce(Hedta)aq]. The kinetic parameters for the Ni2+ or Co2+ ions could be related to the relatively slow (k - 2.6 x 106s 1 for Co2+ and 3.4 x 104 s-1 for Ni2+) water exchange reactions of these ions. The direct attack was interpreted in terms of an intermediate in which one of the carboxylate groups was coordinated to the incoming ion rather than to Ce3+. These reactions were followed by spectrophotometry at 280 nm, where the absorbance of Ce3+aq is much lower than the edta complex. 

Planar-tetrahedral equilibria of nickel(II) complexes were the first spin-equilibria for which dynamics were measured in solution. It had been known that such complexes were in relatively rapid equilibrium in solution at room temperature, for their proton NMR spectra were exchange averaged, rather than a superposition of the spectra of the diamagnetic and paramagnetic species. At low temperatures, however, for certain dihalodiphosphine complexes, it is possible to slow the exchange and observe separate resonances for the two species. On warming the lines broaden and coalesce and kinetics parameters can be obtained. Two research groups reported such results almost simultaneously in 1970 (99,129). Their results and others reported subsequently are summarized in Table V. 

The pH dependence of Ks/Km is similar for step 1 and step 2 reactions as shown in Fig. 26b, but this similarity in the pH curves indicate only that the same titratable groups on the free enzyme and/or free substrate are involved in the two steps. As discussed explicitly by Usher et al. (522) the roles of the two histidines could be reversed and this would make no difference since the ratio of HE EH where these are the two singly protonated species is independent of pH. Similar ks and Ka curves for the two steps would also fail to prove identical roles for the two histidines. Since a pentacovalent species—whether it is a transient activated complex or a more stable intermediate—is common to the various alternatives, pK shifts deduced from ka curves could be the same. Both substrates are monovalent anions with low pK values so that 1 /Km, whether interpreted as an equilibrium binding value or as a function of the kinetic parameters mirroring the total occupancy of all the stable intermediates, could also be the same for both steps. The values for the reverse of step 2 would behave differently since the pj of 3 -CMP, for example, is 5.9. It should also be noted that ks/Km curves should be and are ionic strength dependent (508) in the same way that the His 12 and His 119 pK values are as observed by NMR (280). 

The complex Ru(tpy)(bpy)02 [tpy = 2,2,2"-terpyridine, bpy = 2,2 -bypyridine] oxidizes organic substrates by hydride abstraction or oxo transfer. This complex, and its derivatives, cleave DNA by oxidation of the sugar at the V position and oxidation of guanine. Oxidation at the V position leads to the release of free bases and a furanone product. The kinetic parameters for the oxidation of D-ribose, 2-deoxy-D-ribose, and nucleotides by Ru(tpy)(bpy)02 were determined in phosphate buffer (pH 7). The increased reactivity of DNA as compared to RNA was rationalized on the basis of deactivation of the sugar oxidation product by the polar effect of the 2 -hydroxyl group.160... 

Chiral (nitrosyl)ruthenium(salen) complexes have been found to be efficient catalysts for aerobic oxidative desymmetrization of mc.vo-diols under photoirradiation to give optically active lactols. With the suitable catalysts, high enantioselectivities up to 93% has been achieved. The kinetics of the oxidation depend on the nature of the ligand. On the basis of kinetic parameters and the kinetic isotope effect, it has been suggested that when a ligand with an apical hydroxy group is used, the hydrogen atom... 

Only a few studies have tackled the problem of deriving a detailed kinetic model of the phenol-formaldehyde reactive system, mainly because of its complexity. In recent years, a generalized procedure has been reported in [11,14] that allows one to build a detailed model for the synthesis of resol-type phenolic resins. This procedure is based on a group contribution method and virtually allows one to estimate the kinetic parameters of every possible reaction taking place in the system. 

Benson [15] has created and developed a general method of calculation of the values of thermochemical and kinetic parameters based on a systematic use of the additivity of group properties on the one hand and the activated complex theory on the other. Other methods for estimating a priori kinetic parameters have recourse to structural analogies or semi-empirical correlations. 

Corresponding to the different use of the criteria, a subdivision into two groups appears to be useful. Experimental criteria are needed when the kinetics of the reaction under consideration are still unknown, i.e. neither the type of rate law nor the intrinsic values of the kinetic parameters have yet been identified. This may be the case during an early stage of a laboratory kinetic study when a new reaction is analyzed for the first time. Experimental criteria in general contain only directly observable quantities, i.e. the measured effective rate of reaction as well as some (effective) physical properties of the catalyst and the reaction mixture (R, Z>c, Ac, etc.). Therefore, these can be easily applied. However, experimental criteria suffer from the disadvantage to be sometimes less conservative when more complex kinetics prevail. 

Not all terpene synthases catalyse complex reactions. Isoprene synthase converts DMAPP to the hemiterpene (G5), isoprene (Fig. 5.1), a comparatively simple process involving the ionization of the diphosphate group, followed by double-bond migration and proton elimination (Silver and Fall, 1991). Present in chloroplasts in both stromal and thylakoid-bound forms, isoprene synthase is a homodimer that differs from other terpene synthases in many properties, such as subunit architecture, optimum pH and kinetic parameters... 

Data in Table 19 show the variations In the IMM of PMAA molecules (or poly-(acrylic acid), PAA), when these interact with insulin The polymer complex was se rated from the unreacted molecules. The great decrease in intramolecular motions in PMAA macromolecules as compared to those of PAA in polymer complexes with insulin may be due to hydrophobic interactions between methyl groups of PMAA and non-polar grou K of insulin in aqueous PMAA-insuIin solutions. Kinetic characteristics of intermolecular interactions in the polymer-polymer complexes have also been studied by tlK PL methodThe dependence of kinetic parameters of intermolecular interactions on the structure of interacting chains, their length and the chemical nature of bonds in PC has also been investigated... 

Rate laws and kinetic parameters for substitution reactions at complexes Cp2TiX2 in acetonitrile solution at 298.2 K have been reported (X = halide or alkoxide). Reactivities are discussed in terms of the nature of the leaving group, the entering group, and the non-leaving Cp ligand. A volume of activation of —15 cm3 mol-1 has been determined for the reaction with thiocyanate.1124... 

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