Homodesmotic reaction

Summary Ab initio calculated bond dissociation energies of silicon compounds will be discussed by means of atomic ionization energies and atomic orbital overlap. Ring strain energies of C- as well as Si-rings are estimated by homodesmotic reactions. The hybridization concept is critically examined in the case of silicon compounds. From the most important results a set of basic rules will be presented. 

Ring strain energy can be defined as the energy of the homodesmotic reaction. 

Fig. 3. Calculated ring strain energies per strained angle in silicon and carbon ring compounds using the homodesmotic reaction. The angle a is half of the deviation of the angle between the straight atomic connection lines from the unstrained angle (109.47 deg).

Repulsive cross-ring interactions in cyclobutane (the 1,3-C—C effect ) might in principle increase the TSE of cyclobutane. However, Cremer and Gauss127 showed that the effect is actually negligible for cyclobutane, but leads to a small indirect decrease in TSE for cyclopropane, due to the way the (small) 1,3-C—C interaction of propane enters the TSE through the homodesmotic reaction (3.180). 

U, 3A, 5A -Triphosphinines were also investigated. In the homodesmotic reaction in eq 10 (X = PH2), 21.8 kcal/mol stabilization has been obtained at the MP2/ 6-31G(d) level (cf. with the 26.0 kcal/mol value for 12. 32. 5A -triphosphinine at the same level of the theory). This stabilization is in agreement with the cyclic delocalization stabilization resulting from donor—acceptor-type interactions discussed in section... 

In a homodesmotic reaction, structural elements in reactants and products match, not only according to the number of carbon—carbon and carbon—heteroatom bonds, but also according to the number of hydrogens bound to each carbon and heteroatom. See George, P. Tratchman, M. Bock, C. W. Brett,... 

Thus, the RE determined from the energy of an isodesmic reaction of bond separation is in fact QMRE-like and represents an estimate of various effects of electron delocalization. By contrast, the use of the homodesmotic reaction leads to a Dewar-type RE (75TCA121) allowing the evaluation of the contribution by precisely the cyclic electron (bond) delocalization. 

The energy of the homodesmotic reaction does not exclusively reflect the effect of cyclic (bond) delocalization. The reference structure is hypothetical and one cannot write the equation of a reaction, where a cyclic and an acyclic structure participate, for which the difference between the energies of products and reactants was determined by a single factor, namely, aromatic stabilization (antiaromatic destabilization) (75TCA121). 

The aromaticity of azines is reduced relative to benzene, as is evidenced by the RCI values (83JOC1344) (Table IV) as well as by the RE values calculated from the energies of hydrogen-transfer reactions (89JA4178) (Table VII). For example, in the case of pyridine the MP3/6-31G calculated energy of the homodesmotic reaction (50) equals -1.8 kcal/mol. Since the RE of benzene determined from the hydrogenation enthalpies is 36 kcal/mol, pyridine s RE will, accordingly, be 34.2 kcal/mol. 

The CSE of 1 is determined by comparison with the energy of a suitable reference compound. Theoretically, this has been done by defining homodesmotic reactions such as those in equations 9 and 1084 ... 

Evaluation of homoaromatic stabilization energies by using homodesmotic reactions... 

The derivation of the resonance energies for 27 and 10 reveals that (a) homodesmotic reactions are well suited to compensate for the different electronic effects that hinder the calculation of pure homoconjugated resonance energies, (b) use of a homodesmotic reaction such as 24 requires the inclusion of many reference compounds, which of course can lead to considerable error progression in the calculated reaction energy, and (c) the... 

SCHEME 13. Homodesmotic reaction energies according to ab initio data for 4456 and 1054 and experimental heats of formation for if1 in kcalmol 1... 

HF/DZ/HF/DZ) kcalmoG1 calculated from the homodesmotic reactions are much smaller than that of 530 (AMI) kcalmoG1 for CgoHgo- This may reflect the fact that Sigo is significantly less strained than Cgo-... 

This is indeed the case, and the ASE for 42 (reaction 8, which was termed homo-molecular homodesmotic reaction [170]) dropped from 5.9 kcal mol 1 stabilization in phosphole to 5.1 kcal mol 1 destabilization in 42 (R H, R2 H) and 12.3 kcal mol 1 destabilization in 42 R2 F) [170], This behavior led the authors to conclude... 

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