Orbitals and chemical reactivity

Orbitals are a mathematical concept and are not real entities. However, they provide the basis for the interpretation of both the structure and changes of chemical substances. Indeed, besides the more fundamental concepts of energy and probability briefly discussed in Chapter 1, the orbital concept -which in fact accommodates the concepts of energy and probability in an intimate way - is, perhaps, the concept with the strongest impact in modem Chemistry. In this chapter, an illustration is given of the direct relationships between orbitals and chemical reactivity and between orbitals and spectroscopy. In the latter, excited states are an implicit part of the problem. However, no detailed treatment of the important relation between excited states and reactivity will be performed in this book. 

This discussion will be mainly qualitative. We will begin with a classic example of chemical reactions directly related to the electron density distribution the electrophilic aromatic substitution reactions of the benzene derivatives aniline and phenol  

The substituent groups -NH2 and -OH interact with the tt orbital system of the benzene ring leading to an increase of tt electron density at the ortho and 

As a consequence, electrophilic aromatic substitution reactions occur more easily than in benzene, and with preference for attack at the ortho and para positions. 

It is noted that the correlation above is with the tt electron distribution and not with the overall electron density at each C atom. Indeed, the alternation of IT population is accompanied by an opposing polarization of the a density distribution. For aniMne, the calculated changes in the tt and c contributions to the charge on each C relative to benzene are (ref 91)  

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Frontier Orbitals and Chemical Reactivity. Chemical reactions typically involve movement of electrons from an electron donor (base, nucleophile, reducing agent) to an electron acceptor (acid, electrophile, oxidizing agent). This electron movement between molecules can also be thought of as electron movement between molecular orbitals, and the properties of these electron donor and electron acceptor orbitals provide considerable insight into chemical reactivity. 

Chapter 10, in relation to classical bond orders and bond energies. In the meantime, Chapter 9 deals with the molecular orbital separation, conjugated systems, non-localizable tt molecular orbitals and resonance. In Chapter 11 a brief extension of molecular orbital theory is made to include three categories of systems fullerenes, transition metal complexes, solid aggregates (and band theory). Finally, Chapter 12 mainly illustrates the direct relations between orbitals and chemical reactivity and between orbitals and spectroscopy, with emphasis on electronic transitions and on spectral parameters in NMR spectroscopy. 

This system has its merits and uses - indeed, we shall employ the line notation almost exclusively - but to understand how bonding occurs, and to explain molecular shape and chemical reactivity, we need to use orbital concepts. 

This chapter introduces and illustrates isosurface displays of molecular orbitals, electron and spin densities, electrostatic potentials and local ionization potentials, as well as maps of the lowest-unoccupied molecular orbital, the electrostatic and local ionization potentials and the spin density (on top of electron density surfaces). Applications of these models to the description of molecular properties and chemical reactivity and selectivity are provided in Chapter 19 of this guide. 

The key to the understanding of physical properties and chemical reactivity of 1 is found in the electronic structure of the molecule, which can be described in terms of molecular orbitals (MOs), valence bond (VB) orbitals or its electron density distribution p(r). Numerous investigations have considered the MOs of 12311 and, therefore, one could expect that a review article on cyclopropane appearing in the year 1995 can skip this part by just referring to one of the previous review articles 1-14 20. However, there is considerable confusion among chemists with regard to the appropriate MOs of cyclopropane, which needs clarification. 

R.A. van Santen, E.J. Baerends, in Z.B. Maksic (Editor), Orbital Interaction and chemical reactivity of metal particles and metal surfaces. Theoretical treatment of large molecules and their interactions, part 4, Springer Verlag, 1991, pp. 323-390. 

In order to understand the importance of frontier orbitals in chemical reactivity, Berkowitz [213] studied the frontier-controlled reactions within the purview of density functional theory. It is evident that the directional characteristics of frontier orbitals determine the extent of charge transfer, and soft-soft interactions are frontier-con-trolled. A somewhat similar analysis showed that charge transfer would be facilitated at a place where the difference in local softness of two partners is large [87], It may be noted that Fukui function is obtainable from local softness but the reverse is not true. On the other hand, local hardness suffers from the drawback of ambiguity [87], which allows one to even consider it to be equal to global hardness without disturbing their... 

As much as the Lewis and VSEPR models help us to understand covalent bonding and the geometry of molecules, they leave many questions unanswered. The most important of these questions is the relation between molecular structure and chemical reactivity. For example, carbon-carbon double bonds are different in chemical reactivity from carbon-carbon single bonds. Most carbon-carbon single bonds are quite unreactive but carbon-carbon double bonds, as we will see in Chapter 5, react with a wide variety of reagents. The Lewis model and VSEPR give us no way to account for these differences. Therefore, let us turn to a newer model of covalent bonding, namely, the formation of covalent bonds by the overlap of atomic orbitals. 

These amino reductones described above are usually very unstable reaction intermediate compounds and, therefore, isolation and elucidation of their precise chemical structures by ordinary experimental techniques are rather difficult. However, owing to the recent remarkable progress in computational chemistry, various types of molecular orbital methods are now applicable to obtain needed information about their precise structures and chemical reactivities. For instance, the optimized structure of L-ascqrbic acid, an important acid-reductone in food and biological systems, was obtained by both semi-empirical and ab initio molecular orbital methods (Abe et aL, 1987, 1992). Semi-empirical molecular orbital calculations were also used to elucidate the autoxidation mechanism of L-ascorbic acid (Kurata et aL, 1996a,b). 

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