Qualitative molecular orbital theory of reactions

It has been found possible to extend and amplify QMOT procedures so that they apply to chemical reactions. One of the most striking examples of this was application to unimolecular cyclization of an open conjugated molecule (e.g., cw-1,3-butadiene, closing to cyclobutene). This type of reaction is called an electrocyclic reaction. The details of the electrocyclic closure of cw-1,3-butadiene are indicated in Fig. 14-17. 

If we imagine that we can keep track of the terminal hydrogens in butadiene (perhaps by deuterium substitution as indicated in the figure) then we can distinguish between two products. One of them is produced if the two terminal methylene groups have rotated in the same sense, either both clockwise or both counterclockwise, to put the two inside atoms of the reactant (here D atoms) on opposite sides of the plane of the four carbon atoms in the product. This is called a conrotatory (con ro ta tory) closure. The other mode rotates the methylenes in opposite directions (disrotatory) to give a product wherein the inside atoms appear on the same side of the C4 plane. 

One can also carry out electrocyclic reactions photochemically. The excited butadiene now has an electron in a tt MO that was empty in the ground state. This MO was the lowest unoccupied MO (LUMO) of ground-state butadiene, pictured in Fig. 14-19. One can see that the step to the next-higher MO of butadiene has just introduced one 

LUMO of butadiene (ground state) or HOMO of butadiene (first excited state) 

SOLUTION The HOMO is antisymmetric for reflection through the symmetry plane that bisects the molecule, and the LUMO is symmetric for this reflection. This is the only symmetry reflection plane where the MOs have opposite symmetry, so the transition is dipole-allowed (and is polarized from one side of the molecule towards the other). The group theory approach for this C2v molecule is that the HOMO has Q2 symmetry, the LUMO has b symmetry, their product has b symmetry, and, since x also has b symmetry, the transition is allowed and is x-polarized (where X is colinear with the central C-C bond). A 

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Section 14-9 Qualitative Molecular Orbital Theory of Reactions... 

The first pair of examples we would like to discuss occurs in a field which lends itself naturally to be conquered by theory. Indeed, the past three decades have seen the exploration of mechanistic details of pericyclic reactions as one of the major success stories of computational chemistry. Rooted in qualitative molecular orbital theory, the key concept of... 

This chapter is an introduction to qualitative molecular orbital theory and pericyclic reactions. Pericyclic reactions have cyclic transition states and electron flow paths that appear to go around in a loop. The regiochemistry and stereochemistry of these reactions are usually predictable by HOMO-LUMO interactions, so to understand them we need to understand molecular orbital theory, at least on a qualitative basis. 

Chemists have increasingly used computational chemistry to study aspects of organometallic chemistry. Although Chapter 2 and subsequent chapters make good use of qualitative molecular orbital theory, the ready availability of easy-to-use computational chemistry software and the powerful capability of modem desktop computers allow chemists to effectively model complex systems to obtain minimum energy geometry of molecules, determine transition state energies, and predict the course of chemical reactions, particularly if two or more isomeric products could form. Researchers have modeled entire catalytic cycles, which... 

Thus the backward shift in the CH3 reaction may be partly a mass effect. Qualitative molecular orbital theory is able to account102 for a similarity in the potential surfaces of H + XY and CH3 + XY and their contrast to the halogen atom plus halogen molecule, Z + XY, potential surface. The molecular orbitals are illustrated for the linear reaction complexes in Fig. 22. 

Ab initio and semiempirical computational methods have proved extremely useful. But also needed is a simple conceptual scheme that enables one to predict the broad outlines of a calculation in advance, or else to rationalize a coii5)uted result in a fairly simple way. Chemistry requires conceptual schemes, simple enough to carry around in one s head, with which new information can be evaluated and related to other information. Such a theory has developed alongside the mathematical methods described in earlier chapters. We shall refer to it as qualitative molecular orbital theory (QMOT). In this chapter we describe selected aspects of this many-faceted subject and illustrate QMOT applications to questions of molecular shape and conformation, and reaction stereochemistry. 

Although a separation of electronic and nuclear motion provides an important simplification and appealing qualitative model for chemistry, the electronic Sclirodinger equation is still fomiidable. Efforts to solve it approximately and apply these solutions to the study of spectroscopy, stmcture and chemical reactions fonn the subject of what is usually called electronic structure theory or quantum chemistry. The starting point for most calculations and the foundation of molecular orbital theory is the independent-particle approximation. 

The period 1930-1980s may be the golden age for the growth of qualitative theories and conceptual models. As is well known, the frontier molecular orbital theory [1-3], Woodward-Hoffmann rules [4, 5], and the resonance theory [6] have equipped chemists well for rationalizing and predicting pericyclic reaction mechanisms or molecular properties with fundamental concepts such as orbital symmetry and hybridization. Remarkable advances in aeative synthesis and fine characterization during recent years appeal for new conceptual models. 

Having learnt about the concerted reactions, we can now undertake the theory of these reactions. The development of the theory of concerted reactions has been due chiefly to the work of R.B. Woodward and R. Hoffmann. They have taken the basic ideas of molecular orbital theory and used them, mainly in a qualitative way, to derive selection rules which predict the stereochemical course of various types of concerted reactions. These rules are best understood in terms of symmetries of interacting molecular orbitals. Here are will see some of the most important theoretical approaches and see how they are interrelated. 

Exponents of molecular-orbital theory treat the subject in two fairly well defined ways. One is to apply the theory in a qualitative or even semi-quantitative manner to aid understanding of chemical processes and the other is concerned more with ab initio calculations of molecular properties. Present ill-defined knowledge of ion structures and reaction mechanisms suggest that the latter approach is unlikely to be rewarding. 

The above molecular orbital theory is always widely used either quantitatively by performing explicit calculations of molecular orbitals or qualitatively for rationalizing various kinds of experimental or theoretical data. As nicely shown by Gimarc (1979) in his comprehensive book Molecular Structure and Bonding, qualitative MO theory allows an approach to many chemical problems related to molecular shapes and bond properties. Its most important achievement is the determination of reaction mechanisms by the well-known Woodward-Hoffmann (1970) rules and the general orientation rules proposed by Fukui (1970). 

A related method to interpret the diastereofecial selectivities of the reactions of double bonds has been proposed by Dannenberg and coworkers [8, 13, 14,]. Tins method also relies on the 7t frontier orbitals of non symmetrical molecules, and proposes breaking the symmetry of the n or it orbitals due to polarization induced by the substituents. Application of frontier molecular orbital theory, taking into account only the substrate MOs, gives a qualitative trend of stereoselection in a number of nucleophilic (reductions of carbonyl compounds) and electrophilic reactions. 

Although sophisticated electronic structure methods may be able to accurately predict a molecular structure or the outcome of a chemical reaction, the results are often hard to rationalize. Generalizing the results to other similar systems therefore becomes difficult. Qualitative theories, on the other hand, are unable to provide accurate results but they may be useful for gaining insight, for example why a certain reaction is favoured over another. They also provide a link to many concepts used by experimentalists. Frontier molecular orbital theory considers the interaction of the orbitals of the reactants and attempts to predict relative reactivities by second-order perturbation theory. It may also be considered as a simplified version of the Fukui function, which considered how easily the total electron density can be distorted. The Woodward-Hoffmann rules allow a rationalization of the stereochemistry of certain types of reactions, while the more general qualitative orbital interaction model can often rationalize the preference for certain molecular structures over other possible arrangements. 

The preceding section focused mostly on qualitative description of the dependence of ORR activity on the structure of N4-metaUomacrocyclic complexes and some special modification procedures which can be used to improve activity. In this section, much of the discussion will focus on quantitative decription of the parameters which influence the ORR activity of N4-metallomacrocyclic complexes. Specifically, the dependence of activity on the properties of the central metal ion will be dicussed in relation to the driving force of the reaction. In addition to this, the molecular orbital theory and the concept of intermolecular are used to describe the interaction between oxygen the central metal ion in N4-macrocyclic complexes and how this interaction influences the ORR activity of the complex. 

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