Isodesmic Subject

While it is straightforward to obtain theoretical heats of formation from processes which greatly disrupt bonding, e.g., the G3 recipe, it is also possible to make use of isodesmic reactions together with limited experimental data, or alternatively data from high-level quantum chemical calculations, to estimate heats of formation. Once in hand, these can be used for whatever thermochemical comparisons are desired. The key is to find an isodesmic reaction which is both uniquely defined, and which leads to products with known heats of formation. This is the subject of the present chapter. 

These points are illustrated in more detail for the case of singlet p-benzyne, which has already been the subject of some discussion in preceding chapters. Consider the following three isodesmic reactions that might be used to determine its heat of formation ... 

Several theoretical studies were performed on the FO2 radical. The best and more recent one is that by Francisco et al (56). References to previous work on the subject are given there. The calculation of AH o for the direct reaction proved to be exceedingly difficult and inaccurate. Therefore, the authors studied also some other isodesmic and/or isogyric reactions, namely FO2 + HO —> HO2 + FO... 

The presumed supramolecular oligomers were subjected to concentration-dependent absorption spectroscopic analysis in solution. Fits of the extinction coefficients obtained as a function of concentration to an isodesmic model allowed association constants to be derived (cf. Table 12.1). On this basis, it was noted that... 

The cyclopropenyl anion has been the subject of many theoretic studies, - and the main conclusion is that it adopts a nonplanar configuration which minimizes the interaction of the anionic lone-pair electrons with the C—C jr-bond. Glukhovtsev, Laiter, and Pross carried out a careful study of the cyclopropenyl system at the G2 theoretical level. They showed that these calculations reproduced the available experimental data, such as the enthalpy of formation of the cyclopropenium ion, and then made use of a series of isodesmic reactions to explore the stability of the species ... 

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