Description of bond dissociation energy

It is important in another way—it is the key for the description of bond dissociation energies. 

Semi-empirical models do not provide an adequate description of bond dissociation energies and should not be used for this purpose. Errors are not systematic, as was the case for Hartree-Fock models (bond energies too small) and local density models (bond energies too large). Rather, significant errors in both directions are observed. 

As will be discussed in Chapter 13, calculated energies of one particular class of isodesmic reactions, so-called bond separation reactions, may be combined with experimental or high-quality calculated thermochemical data in order to lead directly to accurate heats of formation. These in turn can be used in whatever types of thermochemical comparisons are of interest. We start our assessment of isodesmic processes with bond separation reactions. Following this, we consider description of bond dissociation energies, hydrogenation energies and acid and base strengths in terms of isodesmic processes, that is, not as absolute quantities but expressed relative to standard compounds. 

H. Basch, P. Aped and S. Hoz, A Valence Bond Description of Bond Dissociation Energy Curves ,... 

Equation 16 is the key equation for the description of bond dissociation energies. Indeed, using the definition of Eq-13, and that of the reorganizational energy,... 

Determination of Bond Dissociation Energies from Kinetic Measurements Macroscopic Description... 

Throughout this work, we will refer to the heats of reactions and the methods used for the kinetic determinations. We have collected background information for discussion in this section. A list of important bond dissociation energies is followed by brief descriptions of the more important kinetic methods used for determining radical reaction rate constants discussed in this review. 

Fig. 6.4. Thermochemical description of Stevenson s rule Dab homol5dic bond dissociation energy of bond A-B, IE ionization energy.

A measure of the ability of an atom within a molecule to attract bonding electrons toward itselP . For a bond between two atoms of different electronegativities, the electron molecular orbital cloud is not symmetric, and the atom with the higher electronegativity attracts the larger proportion of the cloud. The most popular quantitative description was presented by Pauling, who based his scale on bond dissociation energies (measured in kcal per mol). 

It is clear that proper description of the energetics of homolytic bond dissociation requires models that account for electron correlation. Are correlated models also needed for accurate descriptions of relative homolytic bond dissociation energies where the relevant reactions are expressed as isodesmic processes A single example suggests that they may not be. Table 6-15 compares calculated and measured CH bond dissociation energies in hydrocarbons, R-H, relative to the CH bond energy in methane as a standard ... 

Another reactive force field that is dependent on bond-order was developed by van Duin, Dasgupta, Loran, and Goddard [183] for hydrocarbons. The configurational energy is described as the sum of energy contributions from internal modes as well as non-bonding van der Waals and Coulombic interactions, but the parameters of the functions that describe each contribution is dependent upon the bond order of atoms involved in each description. It is assumed that the bond order between an atom pair is dependent on the interatomic separation. While this model has been used to predict bond dissociation energies, heats of formation and structures of simple hydrocarbons, it was not applied to predict condensed phase properties. However, the form of the potential should allow for condensed phase studies. 

To justify such a description within quantum mechanics we are led into a consideration of ionic and covEilent contributions to an approximate wave function. If covalent contributions are minor, the bond is said to be ionic in character, and the electrostatic model is considered to be applicable. Unfortunately, the terms ionic character and covalent character are used with various meanings. This is so, in part, because the rapid development of chemical bond theory has caused a drift of the meanings of these terms over the past two decades. Pauling s definitions, as presented in his book (1585, p. 48), no doubt represent the intent of most workers as of 1940. He concluded that there is a covalent bond between two atoms X and Y if the dissociation energy of X—is the mean of the dissociation energies of X— X and Y— Y. If the dissociation energy of X— Y exceeds this mean, the excess is attributed to additional ionic character of the bond. This criterion furnishes the basis for his scale of electronegativity, and ionic character is inter-... 

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