Bond energy, concept

No symbol has been approved by the IUPAC for dissociation energy in the chemical thermodynamics section [13]. Under Atoms and Molecules, either El or D is indicated. The latter is more common, and IUPAC recommends Do and De for the dissociation energy from the ground state and from the potential minimum, respectively. Because the bond energy concept will be omnipresent in this book and can be explored in a variety of ways, some extra names and symbols are required. This matter will be handled whenever needed, but for now we agree to use DUP for a standard bond dissociation internal energy and DHj for a standard bond dissociation enthalpy, both at a temperature T. In cases where it is clear that the temperature refers to 298.15 K, a subscript T will be omitted. 

The reader may wonder at this point whether bond energies for coordination compounds can be tabulated and utilised in chemical arguments, in the same way as in Main Group chemistry. The complexity of most coordination/organometallic compounds makes this very difficult, and the ways in which the formation of a coordinate bond affects the bonding within the ligand renders the bond energy concept distinctly dubious. Consider, for example, the relatively simple case of hexacar-bonylchromium(O). For the reaction ... 

The utility of the bond energy concept shows that it should be easier to obtain the for the localized and van der Waals type correlations directly and individually when dealing with a large... 

The negative sign means that heat is released by the reaction. Because the bond-energy concept is only approximate, this value is only approximate (the experimental value is 101 kJ). 

Let us summarize briefly at this stage. We have seen that the point of degeneracy forms an extended hyperline which we have illnstrated in detail for a four electrons in four Is orbitals model. The geometries that lie on the hyperline are predictable for the 4 orbital 4 electron case using the VB bond energy (Eq. 9.1) and the London formula (Eq. 9.2). This concept can be nsed to provide nseful qualitative information in other problems. Thns we were able to rationalize the conical intersection geometry for a [2+2] photochemical cycloaddition and the di-Jt-methane rearrangement. 

Chapter 2 discusses the properties of bonds such as bond lengths and bond energies, which provide much of the experimental information on which bonding concepts and explanations of geometry have been mainly based. Again this is a brief summary at a fairly elementary level, serving mainly as a review. No attempt is made to deal with the experimental details of the many different experimental methods used to obtain the information discussed. 

Thermochemistry of cluster compounds. In this short summary of cluster structures and their bonding, a few remarks on their thermochemical behaviour are given, in view of a possible relationship with the intermetallic alloy properties. To this end we remember that for molecular compounds, as for several organic compounds, concepts such as bond energies and their relation to atomization energies and thermodynamic formation functions play an important role in the description of these compounds and their properties. A classical example is given by some binary hydrocarbon compounds. 

Another important point deals with selectivity in the abstraction of -protons on equally substituted carbons. In a iyw-periplanar transition state as described above, the minimization of energy concept implies proton abstraction with a maximum of orbital overlap and a minimum of molecular deformation. Consequently, conformations possessing the more acute dihedral angles for bonds H —C—C—O wiU be favored (Scheme 2l/. 

Spectroscopic, crystallographic, and computational studies on the hydrogen bonding networks of various water and carbohydrate systems have led to the concept of o-cooperativity or nonadditivity. This principle holds that the total hydrogen bond energy of a chain of H-bonds will be greater than the total energies of the individual links [94]. 

The description of bond energies that depend explicitly on the charges of the bond-forming atoms is attractive for the concepts it applies and for its usefulness in the prediction of important thermochemical quantities, such as the energy of atomization or the enthalpy of formation of organic molecules. But its success critically depends on the availability of accurate charge results. 

This concludes the enumeration of the concepts involved in our bond energy theory. 

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