Chemical exchange rate

The exchange of the coordinated aqua ligand of the W(IV) aqua oxo species was qualitatively studied by NMR line-broadening as a function of temperature based on Eq. (26), where the transverse relaxation time of the bound oxygen-17 nucleus is given by 1/T2b. The l/T2Qb represents the quadrupolar relaxation rate and kmi the chemical exchange rate constant... 

Determination of chemical exchange rate constants based on the shapes of spectroscopic lines (frequently NMR resonances) of dynamic processes. 

The dependence of chemical exchange rate constant k on temperature T can be expressed by the Arrhenius equation ... 

Quantitative analysis of exchange spectra directly provides data about the chemical exchange and the cross-relaxation rates. Whereas the chemical exchange rate constants are used directly, the cross-relaxation rates are usually processed further for determination of interproton distances and correlation times. 

Fig. 4.1. Calculated NMR spectra of a system constituted by a nuclear species in chemical exchange between two sites, A and B, as a function of the chemical exchange rate, rM . The chemical shift separation between A and B signals is Aa> = 1000 rad s l. The various situations are (A) R2a = Km, Pa = Pb, (B) R2a = R2b, Pa < Pb-...

Basic Silicate Solutions Dynamics. Exchange reactions between silicates as well as zeolite formation involve condensation and hydrolysis reactions between dissolved silicate species. Therefore, we have extensively studied the dynamics of basic silicate solutions in order to obtain better knowledge of the properties of possible zeolite precursor species. Our first results were published earlier (11). Here we have again used selective excitation Si-NMR experiments, applying DANTE-type (13) pulse sequences to saturate a particular Si resonance belonging to a particular Si site. The rate of transfer of magnetization from this saturated site to other sites is then a measure of the chemical exchange rate between the two sites. 

At the energies required for conformational conversions and other exchange processes which are amenable to study by NMR spectroscopy, the reacting molecules have state densities which are much lower than those of molecules undergoing isomerization and decomposition reactions which are generally found to obey RRKM kinetics. Whether these systems can be modeled with RRKM theory is a question of current interest. Table 8 lists molecules for which pressure-dependent gas-phase chemical exchange rate constants have been obtained. 

For example, in the simple case of 1 1 complex formation, several possibilities of chemical exchange rate may exist (Connors, 1987). Slow exchange possibly involves strongly bound complexes, and fast exchange generally refers... 

The chemical exchange rate(s) between two or more vanadium species can lie in about the same time window as the relaxation rates (lifetime of an excited state in an NMR... 

Saturation transfer NMR experiments [32-34] enable chemical exchange rates to be measured in living tissues in the steady state. Hence, enzymatic activity can be studied... 

Rates measured under EX2 conditions are proportional to the chemical exchange rate, which is in turn proportional to the catalyst concentration. This is not true under EXl conditions where the observed rate reports structural opening, independent of chemical exchange. Therefore, measuring the pH dependence of the observed rate of hydrogen exchange allows one to distinguish between EXl (pH independent) versus EX2 (pH dependent) mechanisms. In studies of native state proteins, the EX2 mechanism usually predominates under neutral pH condi-... 

What is the chemical exchange rate at the temperature at which multiplicity is... 

The relationship between the chemical exchange rate and the temperature is exponential a temperature decrease from 25 to 0°C, which represents the typical conditions used during the labeling and quench step of an HX-MS experiment, respectively, leads to an approximate 14-fold... 

Figure 1.6 The chemical exchange rate, as a function of temperature (°C) for base-catalyzed exchange, was calculated from Equation 1.4 for temperatures from 0 to 30°C and normalized to the chemical exchange rate at 0°C...

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