Isotope exchange rates

Figures 19.6 through 19.11 detail the isotopic exchange rates during water-gas shift for the formate, C02, and Pt-CO bands, in switching from the 12C to 13C label. In all cases, the reactive exchange rates of formate and C02 were virtually identical, implicating the formate species as the likely intermediate to the water-gas shift catalytic mechanism over Pt/Zr02 and PtNa/Zr02 catalysts. The DRIFTS spectra at the top of each figure show the switching of these species from...

The large Fe isotope fractionations predicted to occur between aqueous ferric and ferrous Fe species (Fig. 3) has been investigated experimentally at two temperatures for hexaquo Fe(III) and Fe(II), as well as the effect of CF substitution (Johnson et al. 2002 Welch et al. 2003). The kinetics of isotopic exchange in these experiments was determined using an enriched Te tracer for the ferric Fe phase. The Fe tracer experiments reported in Johnson et al. (2002) and Welch et al. (2003) indicate that 95% isotopic equilibrium between Fellll) and Fe(II)gq occurs within —60 seconds at room temperature (22°C), or within —5 minutes at 0°C. The relatively slower isotopic exchange rates at lower temperatures are expected. 

Derivation of rate equations is an integral part of the effective usage of kinetics as a tool. Novel mechanisms must be described by new equations, and famihar ones often need to be modified to account for minor deviations from the expected pattern. The mathematical manipulations involved in deriving initial velocity or isotope exchange-rate laws are in general quite straightforward, but can be tedious. It is the purpose of this entry, therefore, to present the currently available methods with emphasis on the more convenient ones. 

This section, authored by Charles Y. Huang, originally appeared in Methods in Enzymology, volume 63, as the first part of Chapter [4], pp. 54-75 ("Derivation of Initial Velocity and Isotope Exchange Rate Equations"). All equations and references in this entry are numbered sequentially as they originally appeared. 

ENZYME RATE EQUATIONS (3. Derivation of Isotope Exchange Rate Equations )... 

The following derivations demonstrate that an isotope exchange rate equation obtained by the steady-state treatment can be converted to one of exchange at equilibrium ... 

ISOTOPE EXCHANGE RATE LAW Isotope exchange rate laws,... 

If DB quickly diffuses away ( 2 is large compared to k i), the expression simplifies to feobs = k and is solely dependent on the rate of deuteron abstraction. In other words, the isotopic exchange rate truly reflects the rate of deuteron abstraction. If k2 is relatively small, then the expression simplifies to feobs = and the... 

Isotopic exchange reaction rates and oxygen diffusion coefficients of Pt/M-CZ, S-CZ and R-CZ at 41 are listed in Table V (Dong et al., 2004a). The surface diffusion rate for R-CZ (Rg) is nearly 4 times larger than the bulk diffusion rate or the equilibrium isotopic exchange rate (R ), while it is... 

It will be shown below that with the aid of liquid ammonia solutions of potassium amide, the acidity of hydrocarbons (including even saturated ones) cannot only be detected but also determined quantitatively from measurements of hydrogen isotope exchange rates. 

Comparisons of the isotopic exchange rates of the hydrogen atoms of the 7-methyl group in... 

Fig. 7.27. Dependence of the isotopic exchange rate constants (k) and the stability constants (K) of the complexes R(EDTA) on the atomic number of the rare earths. The vertical bars shown in the figure represent the probable error in the k values. The dotted line describing the behavior of the elements lanthanum, neodymium and europium is from the comparison of exchange reactivity attempted by Fomin.

Isotopic Exchange Rate as a Function of Specific Radioactivity< (154)... 

Fig, 10. Comparison between the C isotopic exchange rates and the hydraulic head rates. 

Reaction of isotopic exchange Rate of exchange at 40 torr pressure /molecules 5 X 10- y cm sec / Activation energy / kcal ymole J Order of reaction With respect to With respect to O, CO, ... 

Adsorption measurements, therefore, are still not very fruitful for the study of the mechanism of isotopic exchange of oxide. On the contrary, the measurement of the isotopic exchange rate in molecular oxygen is the only reliable method for estimation of the rate of adsorption (or desorption) of oxygen in the neighbourhood of adsorption equilibrium. 

Identical experiments carried out with both s-MDH and m-MDH from beef heart resulted in similar changes in the isotopic exchange rates (76). For both enzymes, the data were compatible with a compulsory ordered mechanism involving binary complexes of the MDH s with the coenzyme but not with the dicarboxylic acid substrates (75). The isotopic exchange rates were very similar for m-MDH from pig or beef heart, and the results were qualitatively the same for s-MDH from beef heart... 

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