Reactions, classes theory

The interaction between experiment and theory is very important in the field of chemical transformations. In 1981 Kenichi Fukui and Roald Hoffmann received a Nobel Prize for their theoretical work on the electronic basis of reaction mechanisms for a number of important reaction types. Theory has also been influential in guiding experimental work toward demonstrating the mechanisms of one of the simplest classes of reactions, electron transfer (movement of an electron from one place to another). Henry Taube received a Nobel prize in 1983 for his studies of electron transfer in inorganic chemistry, and Rudolf Marcus received a Nobel Prize in 1992 for his theoretical work in this area. The state of development of chemical reaction theory is now sufficiently advanced that it can begin to guide the invention of new transformations by synthetic chemists. 

The outline of the chapter is the following. In Sec. 2, we give an overview of GH theory and various important limits, as well as an overview of its applications to assorted charge transfer reaction classes. In Sec. 3, we sketch a model development that is quite useful in comprehending the meaning of GH theory. Various MD simulation studies on reaction dynamics are described in Sec. 4. from the perspective of the preceding sections. Sec. 5 sketches some other related developments, while concluding remarks are offered in Sec. 6. 

We now turn to charge transfer reactions involving a quantum particle, an electron or a proton. At first glance, these might seem strange applications of GH theory, since the coordinate of the reactant is quantum, and not classical as in Sec. 2.1. But as described below, the actual reactive coordinate for these reaction classes is classical. 

This review has provided an overview of the studies of pericyclic reaction transition states using density functional theory methods up to the middle of 1995. Since the parent systems for most of the pericyclic reaction classes have been studied, a first assessment of DFT methods for the calculation of pericyclic transition structures can be made. 

More complex electrode processes than those described above involve consecutive electron transfer or coupled homogeneous reactions. The theory of these reactions is also more complicated, but they correspond to a class of real, important reactions, particularly involving organic and biological compounds. 

The elucidation of reaction mechanisms is a central topic in organic chemistry that led to many elegant studies emphasizing the interplay of theory and experiment as demonstrated, for example, by the seminal contributions of the Houk group to the understanding of the Diels-Alder and other pericyclic reactions.38 This reaction class is rather typical for the elucidation of reaction mechanisms. On the experimental side, the toolbox of solvent, substituent and isotope effect studies as well as stereochemical probes have been used extensively, while the reactants, products, intermediates and transition structures involved have been calculated at all feasible levels of theory. As a result, these reactions often serve as a success story in physical organic chemistry. 

The theoretical models discussed above are frequently employed in the description of the kinetics of gas-phase reactions, especially reactions of atoms and free radicals. This class of reactions is of interest in a broader scientific context, and a better understanding of their mechanism is of primary importance for the development of chemical modeling. Free atoms and radicals are very reactive species, which occur in and take part in many different reaction systems. Therefore, a radical reaction usually proceeds in competition with a few parallel or subsequent processes. The kinetic behavior of the reaction system may be very complicated and difficult for quantitative description. Theoretical investigations of the reaction kinetics provide information useful for a better understanding and correct interpretation of experimental findings. Results of ab initio calculations are employed to evaluate the rate constant in terms of the computational methods of the reaction rate theory. 

In summary, we are primarily concerned with two classes of reactions (/) bimolecular reactions with a steep chemical barrier or possibly a steric constraint to reaction, and (2) recombination reactions in which motion of the atoms in the strongly attractive well must be treated. In both instances we assume that only the strongly repulsive solute-solvent and solvent-solvent forces need to be taken into account. We present a type of kinetic theory that is capable of handling more general cases, but the two reaction classes suffice to illustrate the use of the theory without overly elaborate calculations. Our goal is a detailed treatment of the effects of solvent dynamics on the reaction. 

Since a century ago Butlerov formulated the idea of the relation between reactivities of compounds and their structure, a huge amount of experimental results obtained for various compounds and reaction classes and corroborating Butlerov s ideas has been accumulated. Attempts to generalize these results and to provide theoretical interpretation of the rules observed experimentally in terms of electrostatic concepts were made by van t Hoff, Ostwald, J. Thompson, Kassel, and later on by Pauling, Coulson and others in terms of the quantum theory of atomic and molecular structure. These attemps resulted in the partial theoretical interpretation of certain problems connected with the relationship between the reactivities and structures of chemical compounds [19, 20, 105, 106, 107, 148, 149, 408, 525]. However, on the whole, the problem is far from being solved. 

The best way to predict how well a given level of theory will describe a transition structure is to look up results for similar classes of reactions. Tables of such data are provided by Hehre in the book referenced at the end of this chapter. 

Transition state theory calculations present slightly fewer technical difficulties. However, the accuracy of these calculations varies with the type of reaction. With the addition of an empirically determined correction factor, these calculations can be the most readily obtained for a given class of reactions. 

Diacyl peroxides undergo thermal and photochemical decomposition to give radical intermediates (for a recent review, see Hiatt, 1971). Mechanistically the reactions are well understood as a result of the many investigations of products and kinetics of thermal decomposition (reviewed by DeTar, 1967 Cubbon, 1970). Not surprisingly, therefore, one of the earliest reports of CIDNP concerned the thermal decomposition of benzoyl peroxide (Bargon et al., 1967 Bargon and Fischer, 1967) and peroxide decompositions have been used more widely than any other class of reaction in testing theories of the phenomenon. 

A part of the chemical consequences of the cyclic orbital interactions in the cyclic conjngation is well known as the Hueckel rule for aromaticity and the Woodward-Hoffmann rule for the stereoselection of organic reactions [14]. In this section, we describe the basis for the rnles very briefly and other rules derived from or related to the orbital phase theory. The rules include kinetic stability (electron-donating and accepting abilities) of cyclic conjugate molecules (Sect. 2.2.2) and discontinnity of cyclic conjngation or inapplicability of the Hueckel rule to a certain class of conjngate molecnles (Sect. 2.2.3). Further applications are described in Sect. 4. 

In this chapter, we wiU review electrochemical electron transfer theory on metal electrodes, starting from the theories of Marcus [1956] and Hush [1958] and ending with the catalysis of bond-breaking reactions. On this route, we will explore the relation to ion transfer reactions, and also cover the earlier models for noncatalytic bond breaking. Obviously, this will be a tour de force, and many interesting side-issues win be left unexplored. However, we hope that the unifying view that we present, based on a framework of model Hamiltonians, will clarify the various aspects of this most important class of electrochemical reactions. 

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