Molecular orbitals conservation

The Woodward-Hoffmann method [52], which assumes conservation of orbital symmetry, is another variant of the same idea. In it, the emphasis is put on the symmetries of molecular orbitals. Longuet-Higgins and Abramson [53] noted the necessity of state-to-state correlation, rather than the orbital correlation, which is not rigorously justified (see also, [30,44]). However, the orbital symmetry conservation rules appear to be very useful for most themial reactions. 

Molecular Orbital Symmetry Conservation in Transition Metal Catalysis Frank D. Mango... 

In a concerted reaction, orbital and state symmetry is conserved throughout the course of the reaction. Thus a symmetric orbital in butadiene must transform into a symmetric orbital in cyclobutene and an antisymmetric orbital must transform into an antisymmetric orbital. In drawing the correlation diagram, molecular orbitals of one symmetry on one side of the diagram are connected to orbitals of the same symmetry on the other side, while observing the noncrossing rule. 

Fig. 21 Schematic representation of strategies for spin alignment in D/A salts or complexes by application of spin conservation in different electron configurations of interacting molecular orbitals, (a) Typical D/A interaction between two closed-shell D and A, (b and b ) McConnell s proposal, (c) Breslow s extension, (d) Torrance s model, (e) Wudl s model, and (f) Chiang s model for further doping.

Roald Hoffmann, a former coworker of R.B. Woodward and Nobel Prize as well for his contribution to the frontier orbital theory (the famous Woodward-Hoffmann rules concerning the conservation of molecular orbital symmetry), has also emphasised the artistic aspects of organic synthesis "The making of molecules puts chemistry very close to the arts. We create the objects that we or others then study or appreciate. That s exactly what writers, visual artists and composers do" [15a]. Nevertheless, Hoffmann also recognises the logic content of synthesis that "has inspired people to write computer programs to emulate the mind of a synthetic chemist, to suggest new syntheses". 

The interpretation of chemical reactivity in terms of molecular orbital symmetry. The central principle is that orbital symmetry is conserved in concerted reactions. An orbital must retain a certain symmetry element (for example, a reflection plane) during the course of a molecular reorganization in concerted reactions. It should be emphasized that orbital-symmetry rules (also referred to as Woodward-Hoffmann rules) apply only to concerted reactions. The rules are very useful in characterizing which types of reactions are likely to occur under thermal or photochemical conditions. Examples of reactions governed by orbital symmetry restrictions include cycloaddition reactions and pericyclic reactions. 

By referring to the diagrams in Figure 3.1 it may be seen that the orbital <)), transforms as Gg+, and that the orbital < )2 transforms as ou+. That two molecular orbitals are produced from the two atomic orbitals is an important part of molecular orbital theory a law of conservation of orbital numbers- The two molecular orbitals differ in energy, both from each other and from the energy of the atomic level. To understand how this arises it is essential to consider the normalization of the orbitals. 

It will be realized that the values of n and m of A will depend on the metal site symmetry and n will only have even values for states of the same parity. In a frequently overlooked paper Eisenstein [554] tabulated the symmetry classifications of the metal ion and ligand orbitals for most of the point group site symmetries of interest. These classifications are often very useful in constructing a molecular orbital energy diagram. Predictions regarding the number and classification of the excited electronic states can then easily be made with the help of such diagrams. We will, however, resist the temptation to reproduce those tables here, in order to conserve space, as they are easily available. 

Roald Hoffmann, bom Zloczow, Poland, 1937. Ph.D. Harvard, 1962, Professor, Cornell. Nobel Prize 1981(shared with Kenichi Fukui Section 7.3.5) for work with organic chemist Robert B. Woodward, showing how the symmetry of molecular orbitals influences the course of chemical reactions (the Woodward-Hoffmann rules or the conservation of orbital symmetry). Main exponent of the extended Hiickel method. He has written poetry, and several popular books on chemistry. 

In absence of a catalyst, simple olefins are essentially fixed in their bonding configurations reaction paths to interconversions through molecular collisions, fusions, and disassociations are apparently closed because of orbital symmetry restrictions, as proposed by Hoffman and Woodward 8°). Mango 8 has postulated that in the presence of certain transition metal catalysts, these orbital symmetry restraints are lifted, allowing bonds to flow freely and molecular systems to interchange. Thus, the conservation of molecular orbital symmetry is a key function of the catalyst. 

McWeeny has written a tribute to the valence-bond theory pioneers of 1927-1935.362 Shavitt has outlined the history and evolution of Gaussian basis sets as employed in ah initio molecular orbital calculations.363 Hargittai has interviewed Roald Hoffmann (b. 1937)364 of Cornell University and Kenichi Fukui (1918-1998)365 of Kyoto University, who were jointly awarded the Nobel Prize in Chemistry in 1981. Fukui developed the concept of frontier orbitals and recognized the importance of orbital symmetry in chemical reactions, but his work was highly mathematical and its importance was not appreciated until Robert Woodward (1917-1979) and Hoffmann produced their rules for the conservation of orbital symmetry from 1965 onwards.366... 

So we have nE s from eq. (7.1.1), and each E value leads to a set of coefficients, or to one molecular orbital. In other words, n atomic orbitals form n molecular orbitals i.e., the number of orbitals is conserved. 

For many years, pericyclic reactions were poorly understood and unpredictable. Around 1965, Robert B. Woodward and Roald Hoffmann developed a theory for predicting the results of pericyclic reactions by considering the symmetry of the molecular orbitals of the reactants and products. Their theory, called conservation of orbital symmetry, says that the molecular orbitals of the reactants must flow smoothly into the MOs of the products without any drastic changes in symmetry. In that case, there will be bonding interactions to help stabilize the transition state. Without these bonding interactions in the transition state, the activation energy is much higher, and the concerted... 

The highest six occupied molecular orbitals and the LUMO of 89 and 90 are displayed in Figure 4.22. If a planar transition state connects these two molecules, then the MOs along the reaction path are rigorously separated into those that are symmetric and antisymmetric with respect to the molecular plane. The symmetric orbitals cannot mix with the antisymmetric orbitals and therefore the electron occupancy of each set of MOs will not change. The reaction is allowed because of the orbital symmetry conservation. 

Although Otto Diels and Kurt Alder won the 1950 Nobel Prize in Chemistry for the Diels-Alder reaction, almost 20 years later R. Hoffmann and R. B. Woodward gave the explanation of this reaction. They published a classical textbook, The Conservation of Orbital Symmetry. K. Fukui (the co-recipient with R. Hoffmann of the 1981 Nobel Prize in Chemistry) gave the Frontier molecular orbital (FMO) theory, which also explains pericyclic reactions. Both theories allow us to predict the conditions under which a pericyclic reaction will occur and what the stereochemical outcome will be. Between these two fundamental approaches to pericyclic reactions, the FMO approach is simpler because it is based on a pictorial approach. Another method similar to the FMO approach of analyzing pericyclic reactions is the transition state aromaticity approach. 

Criteria and guidelines useful in network elucidation and supplementing the rules derived in this chapter include considerations of steric effects, molecularities of postulated reaction steps, and thermodynamic constraints as well as Tolman s 16- or 18-electron rule for reactions involving transition-metal complexes and the Woodward-Hoffmann exclusion rules based on the principle of conservation of molecular orbital symmetry. Auxiliary techniques that can be brought to bear include, among others, determinations of isomer distribution, isotope techniques, and spectrophotometry. 

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