Aromatic stabilization energy

Naphthalene and other polycyclic aromatic hydrocarbons show many of the chemical properties associated with aromaticity. Thus, measurement of its heat of hydrogenation shows an aromatic stabilization energy of approximately 250 kj/mol (60 kcal/mol). Furthermore, naphthalene reacts slowly with electrophiles such as Br2 to give substitution products rather than double-bond addition products. 

The pKa values for 3-phenyl- and 6-methyl-3-phenyl-2//-thiopyran-S,S-dioxides have been discussed in connection with the aromatic stabilization energies of their corresponding anions (91JOC4218). 

The gas-phase chemistry of borazine B3N3H6 (147) and the conjugate N-protonated acid B3N3H7+ indicates analogies with benzene,189 although the aromatic stabilization energy of neutral borazine is only 30% that of benzene, and the reactivities of benzene and borazine are not similar (Scheme 62). Comparable conclusions were reached when HOMA and Iq aromaticity indices were used.190a... 

The chemistry and properties of heteronins 221 (Scheme 82) have been reviewed.264-266 These compounds are thermally stable and possess delocalized planar molecules with strong diamagnetic ring currents.2673 Schleyer and co-workers calculated by ab initio and density functional methods aromatic stabilization energies as well as other properties of heteronins, and found that only 221 anions with X = N and P are aromatic and planar.2676... 

According to a semiempirical study by Malar, the different polyphospholide anions have 86—101% aromaticities of that of the cyclopentadienide anion. Chesnut and Quin reported on the basis of GIAO NMR calculations using a triple-valence quality basis set that the phospholide anion s aromaticity is 63% that of the cyclopentadienide anion. The aromatic stabilization energy (ASE) obtained by Schley-er et al. from eq 2 (X = P ) was 90% that of the cyclopentadienide anion. 

The disproportionation of borirene to acetylene and diboretene (58) is slightly endothermic (by 5 kcal/mol), whereas the activation barrier of this BH transfer reaction equals 14.6 kcal/mol (86JA3960) The estimation of the aromatic stabilization energy for isodesmic reaction (59) yields values of 47.5 (6-31G //6-31G ) (86JA3960), 47.1 kcal/mol (6-31G //STO-3G), [81JA( 103)2589] which makes about 70% of the stabilization energy... 

An estimate of the aromatic stabilization energy of silabenzene, based on the calculation of the ISE (STO-3G basis set) (78JA6499), concludes that its value is 2/3 of the benzene stabilization energy. Possible ap-... 

The lowering of the aromatic stabilization energy of silabenzene, compared to benzene, leads one to expect that the energy of the antiaromatic stabilization of silacyclobutadiene (253) would be lower than in the case of cyclobutadiene. 

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 ... 

Reactions of this type are characteristic of compounds with low aromaticity. While rare in pyridine, they are favored by the following structural modifications which lower the aromatic stabilization energy. 

Using the appropriate bond-separation reactions, the UF/3-21G aromatic stabilization energies are calculated to be 47.2, 36.4 and 22.5 kcal mol-1 for 22,11 and 21, respectively, compared to 59.0 kcal mol-1 for benzene503 thus the meta-, para- and ortho-isomers have 80, 62 and 38% of the aromaticity of benzene. The different orders of the thermodynamic stability of the three isomeric disilabenzene and of their aromatic stabilization energies... 

The effect of several substituents, i.e. R = F, CH3, SiH3 and BH2, on the structure and the aromatic stabilization energies of RgSig (31a or 32a) was studied by Nagase and the main results are summarized in Table 561a. 

TABLE 5. Substituent effects on the structure and aromatic stabilization energy (ASE) of SigRg (31a or 32a) ... 

Substitution of the Si atoms in Sif)I If, by its higher congeners has a small effect on their aromatic stabilization energies i.e. the ASEs calculated according to equation 14a for... 

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