Isodesmic reactions approach

The isodesmic reaction approach (see Section 4.1) has also been applied to calculation of the resonance stabilization of benzene. 

Isodesmic reactions can be very useful for modeling systems and reactions. However, this approach is not without its limitations as well, which include the following ... 

The most common evaluation of aromaticity via energetic criteria is done using calculations either a type of isodesmic reaction (34) or comparison of two isomers that differ only through the aromaticity of one (3). We were interested in the possibility of evaluating stability experimentally and the electrochemical formation of dications such as 8 was attractive. In this approach, the redox potential for formation of the dication would be compared to the redox potential for formation of dications which could not be antiaromatic. If 8 was antiaromatic, its redox potential should be larger and more positive than that of the reference system. This approach was applied to the evaluation of the antiaromaticity of 9... 

One point of debate in defining the magnitude of the captodative effect has been the separation of substituent effects on the radical itself as compared to that on the closed shell reference system. This is, as stated before, a general problem for all definitions of radical stability based on isodesmic reactions such as Eq. 1 [7,74,76], but becomes particularly important in multiply substituted cases. This problem can be approached either through estimating the substituent effects for the closed shell parents separately [77,78], or through the use of isodesmic reactions such as Eq. 5, in which only open shell species are present ... 

In the isodesmic approach, the heat of formation of the radical is obtained by combining the calculated energy of an appropriate isodesmic reaction involving the radical, e.g. 

A semiempirical procedure based on isodesmic reactions is used to determine unknown heats of formation. The choice of the isodesmic reaction is critical for the validity of the approach. By definition, all the bonds are conserved in number and nature in this type of process. Also, the bond lengths of equivalent bonds in reactants and products should be as similar as possible. The stabihzation energies are based on the type of isodesmic reaction which is given for the influence of amino- and cyano-groups in a captodative radical in equation (3). Only radicals appear as reactants and... 

As mentioned above, different approaches to the evaluation of the stabilization in free radicals do not necessarily lead to identical results. Pasto (Pasto et al., 1987 Pasto, 1988), also performing ab initio calculations, used the isodesmic reaction (4) for the evaluation of substituent effects. The heat... 

Alternate methods, each involving some degree of empiricism, also exist for the determination of AHf. One popular approach is to use isodesmic reactions which are reactions in which the products and reactants contain the same number of bonds of each type. The following is an example of an... 

By choosing a suitable isodesmic reaction, the heat of formation of the new species can be determined from the calculated value of the heat of reaction and the thermochemistry of the remaining species, which must also be known. This approach provides an empirical correction in the form of cancellation of correlation energy that accompany the formation and breakage of specific types of bonds. 

One approach for reducing the errors associated with the prediction of pA a values is to employ an isodesmic reaction. To illustrate with a specific example, it may be very hard to correctly predict the free energy change for the aqueous reaction... 

In order to quantify the stabilizing effect of rehybridization at carbon in small rings, two separate computational tacks have been followed. The first approach attempts to determine the energy of a bond in a single molecule using its electron density, while the second approach makes use of isodesmic reaction energies. 

Another approach for hb evaluation exploits the stabilization energy obtainable by considering thermochemistry of appropriate isodesmic reactions for calculating the isodesmic (from greek lo-oq = equal and Ssqop. = bond) reaction s enthalpy (A/fiso) [191 193]. Reagents and products in such reactions... 

There have been several attempts to analyze these substituent effects using computational approaches. For example, the isodesmic reaction... 

The kinetic analysis approach for the determination of the barriers of the dibenzofuranyl system consisted of reducing a species to ist smallest active unity to allow the calculation of the transition state structures. Further improvements were achieved by using literature data, calculated data of Mebel et al. on the vinyl + O2 system and those of Hadad et al. on the phenyl + O2 system. This approche confirmed the satisfactory accuracy of DFT with isodesmic reactions method. 

One way to describe the extent to which heats of formation depend on bonding patterns is to consider an isodesmic reaction—a reaction in which both the reactants and the products have the same number of bonds of a given type, even though there may be changes in the relationship of one bond to another. For example, consider the hypothetical conversion of n-hexane to 2,2-dimethylbutane. Both the reactant and the product have five C-C and fourteen C-H bonds. The simple bond increment approach would calculate that the heat of the reaction should be 0, but the data in Table 1.6 indicate that the heat of the reaction should be -4.4kcal/mol. Therefore, the heat of an isodesmic reaction is an indication of deviation from the additivity of bond energies. " ... 

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