Homodesmotic reactions calculations

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. 

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).

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 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... 

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-... 

TABLE 1. Strain energies (kcalmol ) calculated using homodesmotic reactions"... 

Hexazine has constituted an intriguing fictitious molecule. It was studied theoretically in order to understand why it cannot be observed experimentally. Glukhovtsev and Schleyer concluded that among Ne possible isomers, hexazine is less stable than a twist open-chain dimer of two azide radicals with C2 symmetry (which, however, is thermodynamically unstable relative to the exothermal decomposition into 3N2 releasing 188.3 kcal/mol). In marked contrast to benzene, hexazine is calculated to be non-planar due to lone-pair repulsion into the cr system. Homodesmotic reactions ... 

George, P Trachtman, M. Brett, A. M. Bock, C. W. Comparison of various isodesmic and homodesmotic reaction heats with values derived from published ab initio molecular orbital calculations, J. Chem. Soc., Perkin Trans. 2 1977, 1036-1047. 

Isodesmic and homodesmotic reactions are frequently used for the study of aromaticity from the energetic point of view. However, the energy of the reactions used experimentally or in calculations may reflects only the relative aromaticity of benzene and not its absolute aromaticity. A new homodesmotic reactions based on radical systems predict an absolute aromaticity of 29.13 kcal mol (121.9 kJ mol ) for benzene and an absolute antiaromaticity of 40.28 kcal mol (168.5 kJ moP ) for cyclobutadiene at the MP4(SDQ)/ 6-31G-(d,p) level.2 ... 

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