Thermochemistry of Stable Molecules

The bond additivity rule is the first-order formalism to estimate thermochemistry of stable molecules. Bond contributions to thermochemical properties for some select bonds are given in Table II. The application of this... 

Eight references on the experimental thermochemistry of fullerenes and 24 references on the theoretical thermochemistry of fullerenes are cited by (a) M. E. Minas da Piedade, Ed., Energetics of Stable Molecules and Reactive Intermediates, NATO Science Series, Series C Mathematical and Physical Sciences, Vol. 535, Kluwer Academic, Dordrecht, The Netherlands, 1999, p. 48. Also see (b) J. Cioslowski, Electronic Structure Calculations on Fullerenes and Their Derivatives, Oxford University Press, Oxford, 1995, p. 154. 

Since free radicals can be viewed as being derived from a (closest) stable molecule by the removal of an atom, again only differences must be sought in establishing their thermochemistry. In addition to the symmetry considerations noted earlier, spin corrections clearly are required to establish the thermochemistry of free radicals. For example, C2H5(derived from = 18) therefore, the entropy of C2H5 can be calculated to be... 

It has already been pointed out that delocalization of electron clouds (that is, the spreading of electronic charge over the region of more than one nucleus) often leads to structural stability. Since the resonance concept is one way of handling delocalization, one would expect that if two or more permissible primary structures can be drawn for a molecule, it would be more stable than a hypothetical molecule of the same type with no delocalization. Stability of molecules can be determined experimentally by measuring heats of chemical reactions a stable molecule will give off less heat in a given reaction than would a less stable molecule of the same type. Now classical thermochemistry has been developed so that heats of... 

Following Bozzelli and Ritter (Lay et al., 1995 Ritter and Bozzelli, 1991), the thermochemistry of free radicals is estimated by adding Hydrogen Bond Increments (HBI) to the energy of the corresponding stable molecule where an H has capped the radical site. The HBI groups are also stored in a functional group tree. 

As we did with electronic structure in Chapter 1, in this chapter on molecular structure we will consider reactive intermediates alongside stable molecules. There are many parallels and interconnections, but also some unique features for reactive intermediates. The same notions of strain and stability can be applied to molecules even if they are reactive, because structural issues are fundamentally the same regardless of whether an atom has an octet or not. Before looking at thermochemistry, however, we must consider a general issue for reactive intermediates. We consider a reactive intermediate to be "unstable", although for all types of reactive intermediates there are special cases that are considered "stable". These are imprecise terms, and in the next section we will seek to clarify their meaning. 

Drs. Larry A. Curtiss, Paul C. Redfern, and David J. Frurip present a tutorial on how to compute enthalpies of formation in Chapter 3. Often a computational chemist will want to know how stable a molecule is. The techniques described in this chapter can answer this question. The authors, who have studied what has been called computational thermochemistry, describe ab initio molecular orbital methods (including the highly accurate and popular Gn methods), density functional methods, semiempirical molecular orbital methods, and empirical methods (such as based on bond energies). These methods are richly illustrated with detailed, worked out examples. 

Now the energy of form A+B can be estimated by electrostatics (Chap. 12), and the energy of the hypothetical completely covalent structure A B can be estimated from thermochemistry (Chap. 9). The stability of the actual molecule AB (its heat of formation) may be measured (also described in Chap. 9). When comparisons are made between the experimental stability of AB and the hypothetical stabilities of the two extreme structures, the molecule itself proves more stable than either extreme form. This, as will be remembered, must be so if the resonance treatment has been used correctly. 

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