Isodesmic reactions strain energy calculations

Fig. 14.5 Isodesmic reactions used to calculate strain energy and the aromatic stabilization energy of oxirene. See text...

A decrease in the strain energy of (105) relative to (104) is also found in 6-31G //3-21G calculations (86ZSK151) respective values determined from the isodesmic bond separation reactions are 113.3 and 135.0 kcal/ mol. 

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... 

According to the MNDO calculations (88MI4), the planar structure of the trigermacyclopropenyl cation (296) also corresponds to a minimum on the PES, whereas the analogous structure of (SnH)3+ (297) is a third-order saddle point. The MNDO calculation of the ISE (291) [isodesmic reaction (88)], with a correction for the strain energy determined from isodesmic reaction (89), shows the aromatic stabilization of (291) to be insignificant (1.8 kcal/mol) ... 

However, in many interesting cases, the requisite experimental data are not available, and the olefinic strain for disubstituted alkenes may be estimated using the calculated energies and the following isodesmic reaction ... 

For many strained hydrocarbons and alkenes, heats of formation are not known. In these cases, computational methodology may be used. For a number of molecules, the total energies determined by ab initio calculations and transformed into heats of formation by special group equivalents may be used (81b). Strain energies have also been obtained via isodesmic reactions, which measure deviations from the additivity of bond energies (224). 

There is no unique isodesmic reaction for a particular problem. For example, to compare the ring strain in oxacyclopropane (oxirane, ethylene oxide) with that in cyclopropane, one might calculate the reaction energy ofthe oxygen exchange reaction (remember that isodesmic reactions do not have to be experimentally achievable) ... 

The use of isodesmic-type reactions (method (b)) to calculate the strain energy of tetrahedrane is shown (calculations by the author) it is instractive to examine... 

The total strain in [3,3]-, [3,4]-, [3,5]-, and [3,6]-prismanes was calculated by homodesmotic reactions, hypothetical reactions with the same number of each kind of bond in reactants and products homodesmotic reactions are a refinement of isodesmic reactions [33], and have been extensively used to estimate strain energies [34,35]. The exothermicity of such a reaction is a measure of the SE of the molecule whose rings are being brokea Consider [3,4]prismane, for which a homodesmotic reaction is shown in Fig. 12.14. The energetics of this reaction at the B3LYP/6-3IG level are (my calculations, a.u. or hartrees, omitting ZPE) ... 

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