Heats of atomization

In order to develop a quantitative interpretation of the effects contributing to heats of atomization, we will introduce other schemes that have been advocated for estimating heats of formation and heats of atomization. We will discuss two schemes and illustrate them with the example of alkanes. Laidler [11] modified a bond additivity scheme by using different bond contributions for C-H bonds, depending on whether hydrogen is bonded to a primary (F(C-H)p), secondary ( (C-H)g), or tertiary ( (C-H)t) carbon atom. Thus, in effect, Laidler also used four different kinds of structure elements to estimate heats of formation of alkanes, in agreement with the four different groups used by Benson. 

Another scheme for estimating thermocheraical data, introduced by Allen [12], accumulated the deviations from simple bond additivity in the carbon skeleton. To achieve this, he introduced, over and beyond a contribution from a C-C and a C-H bond, a contribution G(CCC) every time a consecutive arrangement of three carbon atoms was met, and a contribution D(CCC) whenever three carbon atoms were bonded to a central carbon atom. Table 7-3 shows the substructures, the symbols, and the contributions to the heats of formation and to the heats of atomization. 

Table 7.3. The Allen scheme substructures, notations, and contributions to heats of formation and heats of atomization (values in kj/mol).

Two now classical papers 61MI40400, 62JCS2927) contain almost all the experimental thermochemical data on pyrazole and indazole. Heats of combustion determined by Zimmerman (61MI40400) have been used by Dewar to calculate the heats of atomization (Table 25) (69JA796). Quantum mechanical calculations, carried out by Dewar (69JA796) or Olivella 81JHC1189), gave accurate empirical values. 

Compound Heat of combustion (crystalf Heat of combustion (sasT - (Heat of atomization) exp - (Heat of Heat of atomization) formation calc expf Heat of formation calc. ... 

The D values may be easy or difficult to measure, and they can be estimated by various techniques, but there is no question as to what they mean. With E values the matter is not so simple. For methane, the total energy of conversion from CH4 to C + 4 H (at 0 K) is 393 kcal mol (1644 kJ mol ). " Consequently, E for the C—H bond in methane is 98 kcal mol (411 kJ mol ) at OK. The more usual practice, though, is not to measure the heat of atomization (i.e., the energy necessary to convert a compound to its atoms) directly but to calculate it from the heat of combustion. Such a calculation is shown in Figure 1.11. 

Heats of combustion are very accurately known for hydrocarbons. For methane the value at 25°C is 212.8 kcal mol (890.4 kJ mol ), which leads to a heat of atomization of 398.0 kcal mol (1665 kJ mol ) or a value of for the C—H bond at 25°C of 99.5 kcal mol (416 kJ mol ). This method is fine for molecules like methane in which all the bonds are equivalent, but for more complicated molecules assumptions must be made. Thus for ethane, the heat of atomization at 25°C is 676.1 kcal mol or 2829 kJ mol (Fig. 1.11), and we must decide how much of this energy is due to the C—C bond and how much to the six C—H bonds. Any assumption must be artificial, since there is no way of actually obtaining this information, and indeed the question has no real meaning. If we make the... 

Energy of individual atoms Heat of atomization of real molecule... 

Heats of atomization belong to the most important characteristics of ground states. Unfortunately, the number of conjugated radicals for which experimental data are available is very limited. A heat of atomization is defined as the enthalpy of the reaction... 

For radical cations a quantity, AH , can be defined, the meaning of which is close to the heat of atomization. It is obtained by subtraction of the first ionization potential, I, of a parent hydrocarbon from the heat of atomization, AHa. of that hydrocarbon ... 

With hydrocarbon radicals, a difference between the calculated and measured heats of atomization (34, 61) amounts to about. 2-.3% (Table I). Data of,such an accuracy are utilizable even for calculations of chemical equilibria. A fair agreement was also obtained with a series of radical cations (61). 

Using a semiempirical SCF MO it approximation, the tautomerism of several amino heterocycles has been studied (70JA2929). From the theoretical heats of atomization of the tautomers of 4-aminoimidazole (181a-d), it was suggested that there would be a slight predominence of 1-//-4-... 

Heats of Atomization (kcal mol-1) of Monofluorides by Sanderson s Method"... 

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