Kinetic cycle relative rate measurements

In the absence of detailed kinetic parameters for individual reaction steps, the relative propagation of the aromatic- to olefin-based catalytic cycles can be assessed by determining the rate and selectivity of termination products of these cycles that are predominantly unreactive in subsequent MTH reactions. One possibility is to use the ethylene/isobutane selectivity as a measure to describe the relative rates of propagation for the aromatic- and olefin-based cycles [62], Kinetic studies show that ethylene methylation is at least an order of magnitude slower than other olefin methylation reactions, and isotopic switching studies show that the C incorporation of ethylene matches that of aromatics on various zeolites therefore, ethylene can be considered to be a termination product of the aromatics-based cycle. Isotopic switching studies on H-ZSM-5 also show that the incorporation of atoms in isobutene matches that of other olefins, showing that on H-ZSM-5, isobutene is... 

The thermochemical cycle in Scheme 4 can be used to estimate the effect of one-electron oxidation on metal-hydride acidities. The method is analogous to one that has been extensively used to investigate organic cation radicals [10c]. Eq. 29 shows that measurements of °ox(MH) and °ox(M ) provide relative p a data for metal hydrides and their cation radicals. Absolute values for p a(MH +) are obtained if the acidities of the neutral hydrides are known. The oxidation potentials of 18-electron hydrides can be readily obtained by cyclic voltammetry. In our experience, the waves that are obtained are frequently chemically irreversible, even at rather high scan rates. Consequently, the oxidation peak potentials will be kinetically shifted and represent minimum values for the true °ox(MH) data, the estimates for p a(MH +) represent maximum values, and calculated Ap a are minimum values. 

The pK found in this way may be directly compared with Forster-cycle calculations. However, straightforward utilization of the fluorescence titration method is usually limited to moderately strong photoacids due to partial deactivation processes of the photoacid occurring in very concentrated mineral acid solutions. The most accurate method of finding the pK of a photoacid is by direct kinetic measurements of the excited-state proton dissociation and recombination rates °. However, these measurements are not trivial and are limited to a relatively small number of photoacids where accurate measurement of the excited-state reversible dynamics of the proton-transfer reaction is possible. 

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