Effect dissociation kinetics

The dissociation kinetics of macrocyclic complexes have received considerable attention, especially during investigations of the nature of the macrocyclic effect. Before discussing the dissociation of cyclic ligand species, it is of benefit to consider some aspects of the dissociation of open-chain ligand complexes. 

The effect of structural modification to 22 on the binding efficiency and binding dynamics of hairpin DNA immobilized on metal surfaces were studied using surface plasmon resonance.124,125 The association and dissociation kinetics were analyzed using a sequential model for the binding of the imidazole containing polyamides... 

DR. DAVID RORABACHER (Wayne State University) As Dr. Margerum has just pointed out, the increase in the number of experiments one needs expands dramatically as one encounters kinetic contributions from other species in the reaction mixture. We have recently encountered such a phenomenon in our laboratory which merits some comment within this context. This phenomenon involves an effect of so-called inert anions upon complex formation and dissociation kinetics which we find very disturbing. The implication of this phenomenon is that now, not only is it necessary to vary the concentration of the buffer along with the concentrations of reactants and products, but one must also vary the concentration of every other species in the system. 

O2 and H2 dissociation kinetics are better at higher temperatures (>400 °C), low-cost electrode structures of high surface area Ni and oxides such as spinels or perovskites to replace the very effective, but costly, Pt catalysts have been sought. 

The formation and dissociation kinetics for the complexation of Cu, Zn, Co and Ni with the quadridentate l,4,8,ll-tetramethyl-l,4,8,ll-tetraazacyclotetradecane to give five-coordinate species have been reported.1203 The rate order (Cu > Zn > Co > Ni) is the same as that for H2O exchange, but the rates are much slower, probably owing to conformational changes occurring in the ligand. The dissociation is acid-catalyzed the five-coordinate species are found to be much less kinetically inert than four- or six-coordinate complexes. No macrocyclic effect was observed. 

Computer simulations provide a means of examining the early stages of hydrate formation (nucleation) on a molecular level (Baez and Clancy, 1994 Radhakrishnan and Trout, 2002 Moon et al., 2003, 2005). Computer simulation has also been applied to study hydrate dissociation (Baez and Clancy, 1994 English and MacElroy, 2004) and the effects on dissociation kinetics of external electromagnetic fields (English and MacElroy, 2004). 

This reaction was followed by a second slower reaction that was not characterized in the report. An analysis of the overall results showed that the influence of steric or electrostatic effects on kinetic parameters is not necessarily minor. The dynamics of binding of NO to the diaqua species is mainly tuned by modulation of electron density on the iron center by the porphyrin macrocycle. A volume profile for NO binding based on values ofAV on and A V 0ff of + 1.5 and + 9.3 cm3/mol, respectively, maybe interpreted as an interchange mechanism for the on reaction, as the Fenl-H20 bond is decidedly stabilized (see Figure 7.15a). The volume of activation for the off reaction indicates a less dissociative mode of activation compared with NO release from other porphyrins. 

Cabaniss, S.E. (1990) pH and ionic strength effects on nickel-fulvic acid dissociation kinetics. Environ. Sci. Technol., 24, 583-588. 

Rokitskaya IT, Atonenko YN, Kotova EA. Effect of the dipole potential of a bilayer lipid membrane on gramicidin channel dissociation kinetics. Biophys. J. 1997 73 850-854. 

It is undoubtedly of interest that the substituents in the clathrochelate framework and in apical groups affect the structure and properties of macrobicyclic complexes. In particular, it was noted that N-methylation of the complexes must stabilize the lowest oxidation states of the encapsulated metal ion. In this case, one should take into account the steric effects of substituents whose introduction influences the dissociation kinetics of the sarcophaginates [112]. 

Here, we report on dissociation kinetic studies of the rare earth elements Sm(III) and Eu(III) from PAA and HA. The solution chemistry of both metals is essentially identical, and they are frequently used as analogs for trivalent actinide elements. We have studied the rare earth dissociation kinetics from different size fractions of PAA and HA using ion exchange and ligand exchange methods. The differences in metal labilities for complexes of the two polyelectrolytes and the effect of polyelectrolyte molecular weight on binding times are discussed in the context of possible polyion-cation interactions in these systems. 

What is the effect of kinetic steps that are not isotopically sensitive but stUl mask the isotope effect (e.g., substrate dissociation) ... 

There are two predominant challenges to direct observation of alkanes coordinated to transition metals (1) the short-lived nature of metal/alkane complexes and (2) competition for coordination of the alkane to the metal center. Because of the weak binding energy, alkane coordination is typically short-lived. Thus, fast spectroscopy techniques are required, and these techniques are often coupled with low temperatures in order to slow processes that result in alkane dissociation. In addition to the rapid dissociation of alkanes, most organic substrates will effectively compete (kinetically and thermodynamically) with alkanes for coordination to metals. Thus, the reaction medium is an important consideration since most common solvents are better ligands than alkanes, and attempts to observe alkane coordination have been commonly performed in the gas phase, in hydrocarbon matrices, or in liquid krypton or xenon. Finally, photolysis is generally required to dissociate a ligand at low temperature to create a transient coordination site for the alkane. 

Kinetics of reacting I R = H, OMe with nucleophiles such as the enol of pentan-2,4-dione aromatic amines , phosphorus derivatives and some reactive aromatic compounds , and relative rates with substituted (cyclohexadienyl)Fe(GO)3 cation have been examined. These behave as classically expected, but in contrast to 1-or 2-OMe, a 3-OMe increases rate through its inductive effect. The kinetics agree with electrophilic substitution with the possible intermediacy of n complexes " . Because aryl (N-diene)Fe(CO)3 complexes can rearrange by dissociation into C-aryl derivatives", intermediates could also involve reaction with an N of an indole or a MeO (oxonium cation) of MeO-aromatics. 

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