Chemical bonding and reactivity

The progression of sections leads the reader from the principles of quantum mechanics and several model problems which illustrate these principles and relate to chemical phenomena, through atomic and molecular orbitals, N-electron configurations, states, and term symbols, vibrational and rotational energy levels, photon-induced transitions among various levels, and eventually to computational techniques for treating chemical bonding and reactivity. 

Cyclic systems have frequently been used in studies of chemical bonding and reactivity, reaction mechanisms and a variety of other problems of interest to chemists3. Their utility depends on the changes in the carbon-carbon and the carbon-heteroatom bonds as well as on steric and electronic effects that result from the introduction of heteroatoms into the system. Indeed, the carbon-heteroatom bond length in small rings shows an effective increase with increasing heteroatom electronegativity4, in line with a... 

Conceptual density functional theory (DFT) [1-7] has been quite successful in explaining chemical bonding and reactivity through various global and local reactivity descriptors as described in the previous chapters. The Fukui function (FF) [4,5] is an important local reactivity descriptor that is used to describe the relative reactivity of the atomic sites in a molecule. The FF [4,5] is defined as... 

The OCT has recently been extended to cover many orbital effects in the chemical bond and reactivity phenomena [38, 68-70]. The orbital communications have also been used to study the bridge bond order components [71, 72] and the multiple probability scattering phenomena in the framework of the probability-amplitude channel [73]. The implicit bond-dependency origins of the indirect (bridge) interactions between atomic orbitals in molecules have also been investigated [74],... 

Most of the chemical formulas in this text are drawn to depict the geometric arrangement of atoms, crucial to chemical bonding and reactivity, as accurately as possible. For example, the carbon atom of methane is sp 3 hybridized and tetrahedral, with H-C-H angles of 109.5 degrees while the carhon atom in formaldehyde is sp 2 hybridized with bond angles of 120 degrees. 

Chattaraj, P.K., Chamorro, E. and Fuentealba, P. (1999) Chemical bonding and reactivity a local thermodynamic viewpoint. Chem. Phys. Lett., 314, 114-121. 

A knowledge of molecular heats of formation and chemical bond dissociation energies has always been regarded as fundamental to the understanding of chemical bonding and reactivity. This chapter deals with the extent and reliability of our knowledge of these... 

In principle, we could build up MOs from many sorts of function that can describe an electron density probability distribution, but we have learnt to understand combinations of AOs and to use them in our models of chemical bonding and reactivity. Indeed, if confronted with an MO that was calculated, for instance, using plane waves, most chemists would immediately translate it into a combination of AOs. In the following, we will describe the LCAO approximation and demonstrate some of the effects that are important when AOs interact with each other to form MOs. 

Generally, a VB-treatment is used to teach chemistry (because it is more succinct and less complicated than MO-theory) until cases (like electro-cyclic reactions) are treated that cannot be understood properly within simple VB-theory. This need not be the case, but is usually a good compromise for teaching chemical bonding and reactivity effectively. 

The theoretical description of atoms and molecules has to rely on approximate solutions to Schrodinger s equation. For the standard methods in current use, the starting approximation treats the electrons as if they were independent particles. The advantage of this approach is the ease with which it can be formulated even for very large systems [1]. However, the correlation of electronic motion often has a major role, particularly in chemical bonding and reactivity. The independent-electron approximation does not provide a qualitative model for correlation effects, nor an efficient basis for evaluating numerical contributions from correlation. 

Note that, within the frontier view, the electronegativity and chemical hardness may be considered as two orthogonal (thus independent) chemical descriptors, see the HOMO-LUMO midlevel vs. gap of Eqs. 1.7 and 1.8, and can be therefore further used as 2D realization of the reaction coordinates to build up the chemical orthogonal space (COS) within which the chemical bond and reactivity is described. 

We extend our imderstanding of the concepts of chemical bonding and reactivity learned in Chapter 3 on metals and Chapter 4 on zeolites to catalysis over metal oxides and metal sulfides in Chapter 5. The featmes that lead to the generation of surface acidity and basicity are described via simple electrostatic bonding theory concepts that were initially introduced by Pauling. The acidity of the material and its application to heterogeneous catalysis are sensitive to the presence of water or other protic solvents. We explicitly examine the effects of the reaction medium in which the reaction is carried out. In addition, we compare and contrast the differences between liquid and solid acids. We subsequently describe the influence of covalent contributions to the bonding in oxides and transition to a discussion on the factors that control selective oxidation. 

The many examples included in this chapter clearly demonstrate that far from being dead, VB theory is a vibrant field of research that produces many new methods and key paradigms of chemical bonding and reactivity. It is hoped that this chapter will serve its intended purpose of teaching some elements of this theory. 

Introduces new concepts of chemical bonding and reactivity as the quantum particle of chemical interaction - the bondons and the density functional of chemical action, with their associated principle and quantitative/qualitative realizations, respectively ... 

Formalizes chemical bonding and reactivity in a unitary scenario of chemical indices stages including equalization and fluctuation manifestation of electronic systems, as a natural consequence of their inner quantum nature ... 

About 10 years later, Cramer and Truhlar (2009) presented a more conservative view of the use of KS-orbitals. One should be careful not to stretch the interpretation of KS-orbitals beyond its limits, since KS-orbitals correspond to a fictitious non-interacting system with the same electron density as the correct many-body function. Since the density computed from KS-orbitals is an approximation to the exact density, properties that depend on individual orbitals, with the exception of the energy of the highest occupied orbitals, should be interpreted with care. Nonetheless, many studies published in the literature do employ DFT molecular orbitals to interpret the electronic origins of chemical bonding and reactivity. 

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