Homodesmic reaction

HomoaUyhc peroxy-radicals, alkenyl hydroperoxide cychzation, 213-14 Homodesmic reactions, dioxetanes, 164 Homologous series acyl peroxides, 162... 

Despite the apparent difference in electron distribution suggested by the valence structures 38 and 39, examination of homodesmic reactions leads to the prediction that both structures benefit from aromatic stabilization, the triazine more than the trisilylene. The greater strength of an N—H compared to a Si—H bond tips the scale in favor of the trisilylene structure218. 

Fig. 4.8 Scheme of A H calculation from quantum-chemical calculations using homodesmic reactions... 

The formation enthalpies for the C6()H2n hydrides were calculated from the results of the B3LYP/6-31G calculations using homodesmic reaction (4.4) with fullerene C60, adamantane C10H16 and cyclohexane C6H12 ... 

Salcedo has evaluated the complete family of heterobenzenes 7, 8, 1, 2, and 3 at the BPW91/6-311-G level <2004JST125>. The experimental geometries of 7, 8, 1, and 2 were nicely reproduced. The NICS values varied over only a small range (—7.85 to —8.43), indicating an appreciable aromaticity for all the compounds. On the other hand, a homodesmic reaction showed a regular drop-off in stabilization from phosphorin to bismin. 

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]. 

Accordingly, we recently undertook the determination, by means of homodesmic reactions, of the gas and solid phase AHf of six energetic nitroaromatic compounds for which the experimental AHf(solid) are ambiguous, i.e. two or more values differ by at least 6 kcal/mole [64]. Nitroaromatics should be particularly well suited for modeling by means of homodesmic reactions, since all of the participants in these can have the... 

The situation in true compounds 8, 11 and 12 is similar albeit more complex because of the hyperconjugation [12]. As an illustrative example let us consider benzocyclobutene 11. The corresponding homodesmic reaction for the a-protonation reads ... 

Table I summarizes the calculated strain energies of compound of the type E H , where n is 4, 6, and 8 and E is C, Si, Ge, and Pb, derived from homodesmic reactions (16b). The strain energies of silicon and carbon tetrahedranes are very similar (140 kcal/mol). However, replacement of the three-membered rings by four-membered rings results in a significant decrease in strain energy 140.3 (Si4H4) > 118.2 (SieHe) > 99.1 (SigHg)...

There are six dinitroaniline species with measured enthalpies of formation in the online archive6, but as there are no new measurements or insights since they were last discussed, we forgo mention of them here. We note the two disparate values for 2,4,6-trinitroaniline (also known as picramide) with solid-phase enthalpies of formation of —115.9126 and —72.879 kJmol-1 respectively, because this compound was one of those studied computationally123. The conclusion by these authors, from their use of two different homodesmic reactions and a calculated enthalpy of sublimation, is consistent only with the latter enthalpy of formation. 

The strain energy of 21a (using appropriate homodesmic reactions) is estimated at MP2/6-31G //3-21G( ) + ZPE as 60.3 kcal mol by 6.6 kcal mol-1 lower than in bicyclo[1.1.0]butane122 (at 3-21G( ) this conclusion is reversed121). Boatz and Gordon also find that 21a is by 3.4 kcal mol -1 more stable than the isomeric 23122. 

TABLE 9. Strain energies (kcalmol ) calculated from homodesmic reactions 8 at the HF/6-31G level"... 

Before proceeding further we introduce increments which describe a change in the aromatic ring proton affinities in monosubstituted benzenes. This is most conveniently done by using homodesmic reactions [35]. In this type of hypothetical reaction the number of atoms of the same element, specific types of covalent bonds and approximate hybridization states are preserved being equal in reactants and products. This is very important since it... 

Aditivity of PAs in polysubstituted benzenes is easily obtained by means of homodesmic chemical reactions, if it is assumed that the interaction between substituents is either reasonably small or comparable in the initial base and the final state conjugated acid. Consider for example the PA of 1,2-disubstituted benzene, which can be resolved by employing the corresponding homodesmic reaction in a following way ... 

The ipso protonation deserves a special scrutiny is discussed earlier. Here we show that the additivity rule is operative for the ipso protonation too, if proper reference level is found. Let us consider multiply substituted fluorobenzenes. They are schematically depicted in Fig.7. It is obvious that the proton affinity of benzene cannot serve as a gauge value for the ipso protonation. Instead, we shall employ once again homodesmic reactions and proceed as follows. Protonation at position 1 of 1,2,3-trifluorobenzene will provide an illuminating example in this respect. The corresponding homodesmic reactions read ... 

Molecular orbital calculations at the SCF/3-21G level, and bond separation and super-homodesmic reaction energies suggest there is only minor lone-pair electron delocalization for arsole and stibole <88JA4204>. 

---