Basis sets isodesmic reactions

The determination of the energy of aromatic stabilization of borepin by calculating the energies of isodesmic reactions (62) and (63) with a correction for the strain leads to the value 12.7 kcal/mol (89MI6). The same energy calculated with the same basis set (6-31G ) is for benzene... 

Little experimental data are available concerning the effect of substituents on the energies of cyclopropanes or cyclobutanes. However, one may use computed energies to gain information on the relative effects on cyclopropane and propane via isodesmic reactions. The data obtained using the 6-31G basis set are given in Table 3.59... 

The Hartree-Fock method has the advantage that it can be applied to relatively large molecules, e.g. octanitrocubane, with a reasonably good basis set, such as 6-31G. Since no electronic correlation is taken into account, Hartree-Fock calculations of heats of formation should be attempted only in conjunction with an isodesmic or homodesmic reaction [24]. If suitable ones are chosen, satisfactory results may be obtained [25,26]. On the other hand, an isodesmic reaction produced an HF/6-31G AHf of 166.1 kcal/mole for cubane, while a homodesmic one yielded 133.4 kcal/mole [25]. The experimental value is 148.7 kcal/mole [27], One must also be wary of a fortuitous element the same homodesmic reaction produced an excellent 149.7 kcal/mole with the very small STO-3G basis set [28]. 

Modified Neglect of Diatomic Overlap Parametric Method Number 3 (MNDO-PM3) 3.10.6 The MNDO/d Method 88 89 5.5 5.6 5.4.5 Correlation Consistent Basis Sets Extrapolation Procedures Isogyric and Isodesmic Reactions 162 164 169... 

Description While ab initio calculations of absolute enthalpies are not currently as accurate as GC methods, relative enthalpies of molecules calculated with the same level of theory and basis set can be very accurate, as in the case of isodesmic reactions. An isodesmic reaction is one in which the number and type of bonds are preserved during the reaction. For example, the reaction of acetaldehyde with ethane to form acetone and methane... 

Isodesmic reactions help us to make the most of the method/basis set we decide to use(this decision being guided by the size of the molecules and the computational resources available, i.e. by how expensive the calculations are). They have on occasion being used with fairly high-level calculations, to obtain high-quality results that would require even higher-level methods were direct-subtraction methods to be used [117]. Isodesmic reactions have been discussed and used extensively [109,118]. 

Experimental and theoretical results for the energy of this reaction are shown in Table IX for some representative cases. It may be seen that the STO-3G basis set provides a rather poor description of this process. This is to be expected, since Equation 7 is very far from being an iso-desmic reaction, and the minimal basis set is fully saturated by the chloride ion. The values given by the 4-31G basis set are considerably better, but it is apparent that the theoretical description of Equation 7 is vastly inferior to the treatment of isodesmic reactions presented previously. In a subsequent section we show that these latter results may be used indirectly to provide information on Reaction 7. It is clear from the results presented in Table IX, however, that the energy for heterolytic cleavage of the C-Cl bond in the gas phase is enormous in comparison with the energies associated normally with chemical reactions, although as shown below this picture is modified drastically in the solution phase. 

In summary, ab initio SCF MO calculations with a 6-31G or larger basis set usually give isodesmic and isomerization reaction energies accurate to 5 kcal/mol. The STO-3G and 3-21G basis sets are not reliable for such calculations. The INDO and CNDO/2 methods are useless for such calculations. The MINDO/3 method is not reliable here, which is disappointing, since it was parametrized to reproduce Ai/ data. MNDO, AMI, and especially PM3 are better than MINDO/3, but clearly inferior to ab initio 6-3IG calculations. Of course, MNDO, AMI, and PM3 can handle much larger molecules than can 6-31G ab initio calculations. 

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. ... 

Isodesmic reactions (9) and (10), in contrast, give results that depend only weakly on the choice of theory and basis set. Their ranges are only... 

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