Bond enthalpy term, determination

The most reliable values of Laidler terms that can applied to a wide variety of compounds are those recommended by Cox and Pilcher [89]. They were derived from a consistent database that includes experimentally determined standard enthalpies of formation for hundreds of organic compounds. This lengthy but simple exercise involves the choice of a set of bond enthalpy terms that affords the best agreement between experimental and calculated standard enthalpies of... 

Table 13.2 lists some experimentally determined values for covalent bond enthalpy terms. When we try to interpret the chemistry of the group 14 elements on the basis of such bond energies, caution is necessary for two reasons ... 

Construct an appropriate Hess cycle, bearing in mind that the P—H bond enthalpy term can be determined from the standard enthalpy of atomization of PH3(g). 

Whether a particular metal is known to crystallize in only a close-packed or body-centered cubic form, or in both, we are therefore able to calculate, from its vaporization enthalpy, the bond enthalpy term (M-M) for bonds of a known length d, and so, using the equation (M-M) = Ad -, we can calculate the value of A for that metal. Values of length-energy correlation constants, A, calculated thus are listed in Table 1, with other relevant data. From these we can, in turn, calculate the enthalpies of all of the metal-metal bonds in (homonuclear) cluster compounds of these metals, if their structures, and so their bond lengths, have been determined. 

We discuss bond lengths in the next section, but we defer the discussion of bond angles to Chapters 4 and 5, where we discuss all aspects of molecular geometry. In later sections of this chapter we discuss bond strength in terms of bond enthalpies and force constants, the determination of approximate values for these properties in polyatomic molecules, and the determination and analysis of dipole moments. 

The standard deviation between experimental and calculated heats of reaction are between 0.5 and 1 kcal/mol for those classes of compounds where enough experimental heats of formation are available to allow a full parameterization. For those classes of compounds where insufficient heats of formation are known to allow the determination of all parameters for 1,2- and 1,3-interactions, an estimate can be given for the bond energy terms which are the dominating parameters. Even here, therefore, a reasonable value for the reaction enthalpy is available. 

Even when C—C bond breaking is the rate-determining step, it is the bond dissociation energy (zero point energy see Section 2.9) rather than the enthalpy term that is important. 

In this section we will apply some of the concepts mentioned above to discuss the energetics of a number of relevant elementary reactions. The required bond enthalpy values will be taken from Table 1 and the entropy values from the literature or estimated. It is worth pointing out that 7A,.y terms are often disregarded in similar discussions, mainly because most enthalpy data are obtained by calorimetric methods, rather than by van t Hoff plots. However, they may determine the thermodynamics of organometallic reactions and therefore should always be considered. As mentioned above, there is a very simple method to estimate entropy terms.This method will be used in the following discussion. The temperature will always be referred to 298 K, unless stated otherwise. 

However, the strength of Lewis acid-base interaction can be expressed in energy terms, such as the exothermic molar heat, —for the equilibrium (III) of adduct formation. The enthalpy term is preferred because entropy effects accompanying the formation of coordinative bonds are difficult to determine. Various models have been proposed for the theoretical estimation of the enthalpy term based on molecular properties of reactants and are reviewed in Ref 5. The most significant developments have been the hard and soft acid-base principle of Pearson [6], the E C equation of Drago and Wayland [7], the donor and acceptor numbers of Gutmann [8], and the perturbation theory of Hudson and Klopman [9]. 

Crystallization is a phase transformation for which the free enthalpy A,rG has to be negative (Equation 2.1). The crystal will be stabilized by minimizing the enthalpy term AtrH, which is determined by the interaction of the building blocks. Interactive forces might either be van der Waals or electrostatic forces. In molecular crystals, hydrogen bonding also plays an important role. 

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