Calculation of stabilization energies

It is often assumed that there will be substantial cancellation of errors associated with the calculation of stabilization energies via reactions such as (6.2). However, this is not always the case. In particular, it has recently been shown [21, 34] that stabilization energies calculated for the cyanomethyl and cyanovinyl radicals show large variation with level of theory. For these situations, methods such as UMP2 perform very poorly because errors associated with spin contamination in the reactant and product radicals are very different and do not cancel. 

Parr and Pearson 1301 defined a parameter 17, which they called "absolute hardness" (17 V4[IP-EA]), and calculated 17 for a variety of neutral and ionic Lewis acids and bases possessing from one to four atoms. These authors showed that the qualitative predictions of the HSAB model regarding the relative reactivities of these species toward one another may be obtained using the results from simple calculations of stabilization energies using 17 and electronegativity values. 

Isodesmic and homodesmotic reactions can use either experimental thermochemical data or energies obtained by MO or DFT calculations. There have been many specific reaction schemes and computational methods applied to calculation of stabilization energies. With the above homodesmotic sequence, calculations at the MP4(SDTQ)/6-31G((i,/ ) level give the following stabilization (AF) values. ... 

At best, van der Waals interactions are weak and individually contribute 0.4 to 4.0 kj/mol of stabilization energy. ITowever, the sum of many such interactions within a macromolecule or between macromolecules can be substantial. For example, model studies of heats of sublimation show that each methylene group in a crystalline hydrocarbon accounts for 8 k[, and each C—IT group in a benzene crystal contributes 7 k[ of van der Waals energy per mole. Calculations indicate that the attractive van der Waals energy between the enzyme lysozyme and a sugar substrate that it binds is about 60 k[/mol. 

Charge-transfer complexes between heteroaromatic five-membered ring compounds and tetracyanoethylene have been studied in solution at 20°C.22 Spectra, stability constants, and empirical calculations of ionization energies... 

The reactant R2 can also be considered to be a solvent molecule. The global kinetics become pseudo first order in Rl. For a SNl mechanism, the bond breaking in R1 can be solvent assisted in the sense that the ionic fluctuation state is stabilized by solvent polarization effects and the probability of having an interconversion via heterolytic decomposition is facilitated by the solvent. This is actually found when external and/or reaction field effects are introduced in the quantum chemical calculation of the energy of such species [2]. The kinetics, however, may depend on the process moving the system from the contact ionic-pair to a solvent-separated ionic pair, but the interconversion step takes place inside the contact ion-pair following the quantum mechanical mechanism described in section 4.1. Solvation then should ensure quantum resonance conditions. 

Response of Stabilization Energy Calculated through Local HSAB Model... 

According to ab initio calculations of the energies of the homodesmotic (86) and the hyperhomodesmotic (87) reactions (85CC1121), the aromatic stabilization energy of (286) amounts to, respectively, 51 and 48% of that of benzene ... 

There are five dihydropyrimidines (455)-(459). Most of those known have either the 1,2- or the tautomeric 1,4- or 1,6-dihydro structures. Gaussian 70 ab initio calculations of the energy of unsubstituted dihydropyrimidines yielded the following order of stability (457) > (456) > (455) > (458) > (459). The results agree with the experimentally observed behavior of these compounds... 

C4H6(g). The experimental value of the heat of formation is 26.3 kcal. Calculate the stabilization energy of 1,3-butadiene. 

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