Exotic kinetics

A number of autoxidation reactions exhibit exotic kinetic phenomena under specific experimental conditions. One of the most widely studied systems is the peroxidase-oxidase (PO) oscillator which is the only enzyme reaction showing oscillation in vitro in homogeneous stirred solution. The net reaction is the oxidation of nicotinamide adenine dinucleotide (NADH), a biologically vital coenzyme, by dioxygen in a horseradish peroxidase enzyme (HRP) catalyzed process ... 

Exotic Kinetics Oscillating Reactions in the Troposphere (J. Phys. Chem. A 2001, 105, 11212-11219. "Steady State Instability and Oscillation in Simplified Models of Tropospheric Chemistry")... 

Traditional reaction kinetics has dealt with the large class of chemical reactions that are characterised by having a unique and stable stationary point (i.e. all reactions tend to the equilibrium ). The complementary class of reactions is characterised either by the existence of more than one stationary point, or by an unstable stationary point (which could possibly bifurcate to periodic solutions). Other extraordinarities such as chaotic solutions are also contained in the second class. The term exotic kinetics refers to different types of qualitative behaviour (in terms of deterministic models) to sustained oscillation, multistationarity and chaotic effects. Other irregular effects, e.g. hyperchaos (Rossler, 1979) can be expected in higher dimensions. 

Csaszar, A., Erdi, P., Jicsinszky, L., Toth, J. Turanyi, T. (1983). Several exact results on deterministic exotic kinetics. Z. phys. Chem. (Leipzig), 264, 449-63. 

Rossler, O. E. (1974b), A synthetic approach to exotic kinetics (with examples). In Physics and mathematics of the nervous system. Lecture Notes in Biomathematics, Vol. 4, eds M. Conrad, W. Giittinger, M. Delcin, pp. 546-82. Springer Verlag, Berlin. 

Toth, J. (1978). What is essential to exotic kinetic behaviour React. Kinet. Catal. Lett., 9, 377-81. 

We have remarked that a temperature of zero on the absolute temperature scale would correspond to the absence of all motion. The kinetic energy would become zero. Very interesting phenomena occur at temperatures near 0°K (the superconductivity of many metals and the superfluidity of liquid helium are two examples). Hence, scientists are extremely interested in methods of reaching temperatures as close to absolute zero as possible. Two low temperature coolants commonly used are liquid hydrogen (which boils at 20°K) and liquid helium (which boils at 4°K). Helium, under reduced pressure, boils at even lower temperatures and provides a means of reaching temperatures near 1°K. More exotic techniques have been developed to produce still lower temperatures (as low as 0.001°K) but even thermometry becomes a severe problem at such temperatures. 

Due to recent developments in synthesis, the preparation of nanocrystalline polymorphs, which are usually unstable as bulk phases, has been achieved for several materials such as ZrC>2, Ti02 and various perovskites. The appearance of these exotic materials does not necessarily mean that they are thermodynamically stable, since the kinetics (templates and surfactants) are probably more important for the processes than the thermodynamics. Adsorption of water may also play an important role as in the case of alumina, but in the data given in Figure 6.19 the effect of water has been accounted for [25]. 

For the rational design of transition metal catalyzed reactions, as well as for fine-tuning, it is vital to know about the catalytic mechanism in as much detail as possible. Apart from kinetic measurements, the only way to learn about mechanistic details is direct spectroscopic observation of reactive intermediates. In this chapter, we have demonstrated that NMR spectroscopy is an invaluable tool in this respect. In combination with other physicochemical effects (such as parahydrogen induced nuclear polarization) even reactive intermediates, which are present at only very low concentrations, can be observed and fully characterized. Therefore, it might be worthwhile not only to apply standard experiments, but to go and exploit some of the more exotic techniques that are now available and ready to use. The successful story of homogeneous hydrogenation with rhodium catalysts demonstrates impressively that this really might be worth the effort. 

There remains one objection—of a less precise kind but felt by many chemists. It is that third-order kinetics as embodied in the representation of step (1) are intrinsically objectionable. If the equations had to be interpreted as representing elementary steps, this would be a weightier consideration, but it has also been asserted that the oscillatory properties of certain other model schemes collapse completely (King, 1983 Gray and Morley-Buchanan, 1985) if the third-order steps therein are replaced. Accordingly it is most desirable to establish whether oscillations and other exotic behaviour arising from a cubic rate-law of the form k ab2 can also arise from a series of successive second-order or bimolecular steps. Similar interests have been expressed previously by Tyson (1973) and Tyson and Light (1973). 

In addition, this procedure was quite tedious and time consuming. Therefore, in recent years when physical methods for assaying molecules in mixtures—methods such as nuclear magnetic resonance (NMR), gas chromatography, and others—have become available, a renaissance in the study of redistribution reactions has taken place. These methods allowed a rapid, quantitative, and precise determination of all of the reaction products present in a mixture. Also, equilibrium reactions could be carried out in much smaller sample sizes, thus permitting the study of exotic, hard-to-obtain compounds. Redistribution reactions—the kinetics as well as the equilibria—can now be measured directly in sealed NMR tubes. Furthermore, the relatively recent widespread availability to chemists of high-speed computers, in addition to these modern analytical tools, has facilitated the use of the appropriate mathematics even when highly complicated. 

Solvated electrons were considered to be exotic over several decades and that is why comparatively little time was devoted to their study. However, the situation drastically changed early in the 60 s when the crucial role of solvated electrons in radiation chemistry was reliably ascertained. This triggered numerous theoretical and experimental studies into the structure and properties of solvated electrons and their solutions, the pathways of formation of solvated electrons, the kinetics and the mechanism of reactions involving their participation. The findings of these studies have been summarized in several monographs and in the proceedings of international conferences dedicated to the physico-chemistry of solvated electrons (the last, 6th Weyl Symposium was held in 1983) therefore, these will not be spjecific-ally considered in this overview. We shall juxtapose these with electrochemical data, of course. The same relates also to electron photoemission from metal into solution, to which monographs are devoted. 

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