Activation Parameters Directly from Reaction Rates

A. Activation Parameters Directly from Reaction Rates.. . . 166... 

For isolated HEI such as dioxetanes and other cyclic and linear peroxides that act directly as reagents in the excitation step, kinetic studies lead to rate constants and activation parameters for this excitation step and conclusions with respect to the mechanism of chemiexcitation can be obtained from the structural and conditional dependence of these parameters. For complex CL systems, in which the HEI is formed in rate-limiting steps prior to the excitation step, the kinetic parameters of this essential reaction step can only be obtained indirectly (see below). 

As seen from Fig. 10, the non-exponential drop in the intensity of fluorescence for Nh-inverse kinetic isotope effect is observed as distinct from the process of spontaneous deactivation, for which a normal isotopic effect (td > ih) is observed. Note that the possibility of the abnormal isotopic effect for electron tunneling reactions follows directly from the theoretical concepts set out in Chap. 3, Sect. 6. The mean values of the parameter / obtained from experiments with various concentrations of CC14 proved to be p = (0.240 0.010) M 1 for Nh d8 and j = (0.205 0.010) M l for Nh. As the effect of the nuclear motion on W R) must be reflected more in the value of ve than in that of ae it seems natural to connect the difference observed between the values of P for Nh and those for Nh-d8 with the change in the parameter ve. At the value of ae 1 A typical of tunneling reactions, the difference observed in the values of P corresponds to an approximately 2.5-fold increase of ve upon naphthalene deuteration. With an increase in temperature from 77 to 140 K, the parameter / remained virtually unchanged, although the time, t, for spontaneous deactivation was markedly reduced. Thus, tunneling reaction (14) proceeds via a non-activated mechanism. 

From Fig. 27 the activity seems to increase by decreasing cluster size. In fact if we want to compare the intrinsic activity of clusters with different size the TON is not necessarily a pertinent parameter. Indeed, if the reaction rate depends on the pressure of at least one reactant, the TON would not take into account the fact that the total flux of one reactant joining the clusters is not solely given by the pressure of this reactant, but we have also to consider the flux of the molecules physisorbed on the substrate. This contribution can be up to 10 times larger than the direct flux and it is strongly cluster size dependent (see Section 4). In that case the right parameter to compare the intrinsic activity of the different clusters is the reaction probability (of NO or CO). It is equal to the consumption rate of one reactant divided by... 

Beyond the instantaneous rate measurements described above, determining how enzyme activity varies with substrate concentration can provide useful information about enzyme capacity and the rates that are Hkely to be observed under different environmental conditions. These kinetic measurements probably work best in in vivo assays of exo- and ectoenzymes where substrate concentrations can be measured directly from the external environment and there are no metabolic intermediary pools or reactions to compHcate the picture. Kinetic measurements also have limitations as there can be bottle effects, diffusion boundary layers around organisms can be important, and it is often difficult to make measurements at the low substrate end-members in aU but the most oligotrophic environments. Further, kinetic parameters are physiological variables themselves, dependent on the preconditioning of cells and so can vary widely even in the same organism, across environments (see Chapter 7 by MulhoUand and Lomas, this volume). 

The most direct information on the isomerization processes has been gained from NMR studies of the phosphine complexes [NiX2L2]. The rates of some of the planar-to-tetrahedral interconversions are similar to the NMR time scale. In these cases, separate signals can be observed for the two isomers at low temperatures, but at the high-temperature limit, averaged resonances are seen (102-104). These observations allow reaction rates and activation parameters to be calculated. 

---