Theory of molecular orbitals

The catalytic reactions occur on the surface, after adsorption and dissociation of molecules, and products leave after desorption. Molecules may form or break up links, due to the interaction between the molecules and the surface atoms, with formation of intermediates and complexes. Due to this interaction, there are overlapping bands of electrons from the metal surface and the molecule. With the superposition of bands, there are variations of electron density. 

The dissociation and adsorption and chemical reaction depend on the activation energy barrier, according to Horiuti and Polanyi [8], which is related to the free energy changes during the reaction. The correlation deduced by Evans and Polanyi [9] is given by the following expression  

The energy barrier is shown in the Lennard-Jones potential energy graphical representation (Eig. 5.7), indicating how the energy varies with distance between molecules or atoms, as, for example, for the reaction A + B R. 

Initially, the molecule A moves in the direction B, as shown in Eig. 5.8. If the distance between A and B is great, these molecules are attracted, and when they approach they are repelled. The electron cloud of A tends to overlap on cloud B, which causes electron repels (Eig. 5.8). This repulsion causes the molecules to not come closer, as van der Waals forces. With the time the adsorbed molecules form complex that decomposes only when overcoming the energy barrier breaking the link AB to form a new bond and the final product R [3, 4]. 

Principles of Adsorption and Reaction on Solid Surfaces, John Wiley Sons (1996), 7] 

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Practitioners of quantum chemistry employed both the visual imagery of nineteenth-century theoretical chemists like Kekule and Crum Brown and the abstract symbolism of twentieth-century mathematical physicists like Dirac and Schrodinger. Pauling s Nature of the Chemical Bond abounded in pictures of hexagons, tetrahedrons, spheres, and dumbbells. Mulliken s 1948 memoir on the theory of molecular orbitals included a list of 120 entries for symbols and words having exact definitions and usages in the new mathematical language of quantum chemistry. 

Robert S. Mulliken, a founder of the theory of molecular orbitals. 

The history of benzene is one of the most intriguing in science. It started in 1825 with the isolation of benzene by Michael Faraday from the condensed phase of pyrolyzed whale oil. Its planar cyclic structure was first proposed in 1861 by the Austrian physicist and physical chemist Johann Josef Loschmidt [1—5]. However, it was only fully understood some 70 years later, around 1930, with the advent of the modem theories of aromaticity, i.e. the theory of molecular orbitals (Hiickel s theory) [6-8] and the theory of resonance [9-12]. 

Contrary to all former theories, in the theory of molecular orbitals the emphasis was on component (A), component (B) was adopted in a generalized form (many-center nonlocalized molecular orbitals), and component (C) was considered to be a mere incidental characteristic of chemical combination. Mulliken believed that the... 

These preliminary thoughts were further extended in the thesis submitted to the Fellowship Examination at Trinity College in 1934. Its title was "The Electronic Structure of Molecules from the Standpoint of the Theory of Molecular Orbitals." The... 

Th6 theory and the analysis of calculational schemes of quantum chemistry have been dealt with in detail in a number of books [1-6] and review articles [7-12]. Here we give only a brief account of the main principles of the general theory of molecular orbitals (MO) that provides the basis for constructing the most important nonempirical methods of quantum chemistry. 

If we regard each electronic formula as dynamic rather than static, there is little to be gained by emphasizing the concept of resonance hybrids. Furthermore, the detailed theory of resonance has come under heavy fire. It is claimed by A. Burawoy that .. . the interpretation of polyatomic structures does not require the hypothesis of resonance among several idealized valence-bond structures, a speculative application of the quantum-mechanical conception of resonance, which is in disagreement with numerous facts. In view of this controversy, we shall avoid detailed application of the theory of resonance. The theory of molecular orbitals, an alternative treatment, is too complex for the present discussion. 

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