Isotope partitioning

In CLTST there appears a kinetic isotope effect owing to the difference in partition functions in the initial state [see eq.(2.12)], and at 2Pf < o > I5... 

Further evidence regarding the mechanism was provided by LynnandBoums643 , who found a pH-dependent carbon-13 isotope effect in the decarboxylation of 2,4-dihydroxybenzoic acid in acetate buffers. The dependence was interpreted in favour of the A-SE2 mechanism, for an increase in acetate concentration would increase kL t and hence partitioning of the intermediate so that k 2 becomes more rate-determining. 

MMl represents the mass and moment-of-inertia term that arises from the translational and rotational partition functions EXG, which may be approximated to unity at low temperatures, arises from excitation of vibrations, and finally ZPE is the vibrational zero-point-energy term. The relation between these terms and the isotopic enthalpy and entropy differences may be written... 

The small isotope effects found most likely arise from the reversibility of step 1 by a partitioning effect The rate at which ArHY reverts to ArH should be essentially the same as that at which ArDY" " (or ArTY ) reverts to ArD (or ArT), since the Ar H bond is not cleaving. However, ArHY" should go to ArY faster than either ArDY or ArTY", since the Ar—H bond is broken in this step. If 2 > 1. this does not matter since a large majority of... 

A DIFF relates a specified body component isotopic composition to the various compositions of a specified and complete set of dietary components. The complete diet must be accounted for in the DIFF, but it can be partitioned in any way that seems sensible for example, into individual amino acids or into protein, carbohydrate and lipid, etc. 

Phillips DL, Gregg JW. 2001. Uncertainty in source partitioning using stable isotopes. Oecologia 127 171-179. 

In summary, a key aspect to the utility of U-series isotopes in the study of arc lavas is that whereas Th and Pa are observed and predicted to behave as relatively immobile high field strength elements (HFSE), Ra and (under oxidizing conditions) U behave like large ion lithophile elements (LILE) and are significantly mobilized in aqueous fluids. Fluid-wedge interaction will only serve to increase these fractionations. Just how robust the experimental partition coefficients are remains to be established by future experiments. 

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