Bond energy intrinsic

Now we turn our attention to the intrinsic bond energies, s i. Two approaches will be considered (1) a derivation making use of Eq. (10.2) and (2) a derivation rooted in Eq. (4.47). But first, we deal with the nonbonded part, Eq. (10.3), and examine future possible simplifications regarding this energy contribution. 

Summa summarum, Eq. (10.12) is certainly not the most efficient one for routine calculations of intrinsic bond energies. The reason is obvious—it lies with the partial derivatives (dski/dZk)p, which must be carried out at constant electron density p, meaning that this difficult calculation has to be made for each new s/ i, which is unpractical. 

The approximate validity of Eq. (12.9) highhghts the leading role played by the reorganizational energies but also vividly demonstrates the importance of using the correct intrinsic bond energies, whose values range from 69.63 to 89.1 kcal/mol... 

The link between dissociation and intrinsic bond energies evidently transcends academic curiosity because of its important down-to-earth practical aspects chemistry, after all, is an exercise of making and breaking chemical bonds, more often than not influenced by the environment in which the reactions occur. Ski is observation, is action. 

The comparison between dissociation and intrinsic bond energies is intriguing, to say the least. It suggests (Fig. 12.1) that those bonds that contribute more to thermochemical stability are the ones that break up more easily. This is certainly a point requiring clarification ... 

Alkane carbon atoms satisfy the charge-NMR shift correlation [Eq. (6.8)]. With the alkylamines, things could be different because of a possible extra effect due to the presence of the nitrogen atom a-carbons should perhaps be compared only among themselves, and so should the /3- and y-carbons. The S-carbons, in contrast, which are sufficiently separated from the nitrogen center, could probably be treated as if they were part of an alkane. This point has been examined as follows for the —C 2—H2—NH2 motif, focusing on the dissociation and intrinsic bond energies, Dc Cp and sc Cp, respectively. 

Straightforward applications of the theory are presented in the atom-by-atom approach, as exemplified in Table 15.4, using charges deduced from NMR shifts, Eq. (6.8) for the carbon atoms, and Eqs. (6.12)-(6.14) for the nitrogen atoms. (CN bond dissociation energies and comparisons with the corresponding intrinsic bond energies are described in Chapter 12 for both alkylamines and selected nitroalkanes.)... 

The intrinsic bond energy reflects the energy required to remove a fragment adsorbed on the Pt4 cluster to the gas phase. It is given for hydrogen bonded on Pt4by ... 

The heat of adsorption, Eads and intrinsic bond energy Eint for Pt-H are given in Table 1. Eads is 70 kJ/(mol H2) more exothermic in the case of the basic support compared to the acidic support. Analogous to the heat of adsorption, the intrinsic bond strength is increased by 35 kJ/(mol H) for the basic supported. 

Table 1 The heat of adsorption (EadS) and intrinsic bond energy (Ej t) for the adsorption of H2 and CHn in the atop, bridged (br.) and 3-fold hollow site (3-f.) on Pt4/F20 or Pt4/Na20.

The intrinsic bond energy Ejnt for Pt-CH3 is smaller for the acidic support (Table 1), just as was observed for the Pt-H bond however, the difference is much smaller for CH3 16 kJ/(mol CH3) vs. 35 kJ/(mol H). The heat of adsorption for CH3 is 50 kJ/mol smaller in the case of an acidic support compared to a basic support. 

Figure 7 The relative intrinsic bond energy as a function of the type of Pt-adsorbate bonding with the acid/base properties of the support as independent parameter. The average E t is set to zero for each adsorbate.

Not an intrinsic bond energy. At MP4SDTQ/6-31G //6-31G(d) from Reference 56. 

The electronic energy, AL/ i, is of interest mainly in connection with bonding theory since it is not an experimentally accessible quantity. A second quantity of this type is the intrinsic bond energy, the difference in energy between the atoms in the molecule and the separated atoms in the valence stafe, i.e., with all of the atoms in the same condition (with respect to spin and hybridization) as in the molecule. It is a measure of the strength of the bond after all other factors except the bringing together of valence state atoms have been eliminated (cf. the discussion of methane, Chapter 5 and McWeeny, R. Coulson s Valence, 3rd ed. Oxford University Oxford, 1979 Chapter 7). 

The final step then is to combine the atom in the valence state, V4, with the four X atoms (which have also been suitably promoted to a valence state) to give the product MX4(g). The energy released in this final step is sometimes called the intrinsic bond energy. For CH4 and SiH4 the various energies... 

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