Isodesmic reaction limitations using

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 ... 

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. 

As with the stable hydroperoxide molecules above, the isodesmic reactions for the unsaturated alkoxy and peroxy radicals resulting from C—OOH, CO—OH and COO—H bond cleavage reveal that enthalpies on a number of vinyl and ethynyl, alkoxy and peroxy radicals are needed for use as reference species. Limited data are available for these radical species enthalpies [47] and this data is listed Appendix A. 

In theory there is no limit to the number of functions that could be included in a basis set. Indeed, theoreticians often refer to the "Hartree-Fock limit as the energy that would be calculated for a system if an infinite number of basis functions were included in the calculation. That energy would still be greater than the energy of the molecular system, however, because the HF method cannot account for the additional stabilization to be gained through electron correlation. Nevertheless, HF calculations can provide very useful information if the electron correlation errors cancel when one compares one system to another. This is more likely to be the case for isodesmic reactions, in which types of bonds are the same in both the reactant and product. ... 

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