Vertical resonance energy

UV spectroscopy, 4, 178, 179 vertical resonance energy, 4, 191 Vilsmeier-Haack formylation, 4, 221, 222 3H-Indoles... 

On the other hand, we should perhaps not be too surprised if values of fi derived from different kinds of measurements do not agree exactly, and indeed this is the case. We have seen above that/ obtained from the vertical resonance energy of benzene is about 37 kcal/mol, whereas Platt first showed that the best overall fit to the spectra of benzene and other unsaturated hydrocarbons was obtained in the framework of the Huckel approximation by taking / to be 55-60 kcal/mol. This high value, which has subsequently been widely adopted to estimate actual differences in energies between MOs, is generally known as the spectroscopic value of / . 

Table 3. Vertical Resonance Energies (Bo) for Delocalized Species...

Vertical Resonance Energies of Aromatic and Antiaromatic Species... 

The largest difference in vertical resonance energy occurs for aromatic and antiaromatic species with an... 

Figure 5. The cr-energy curve (a) and the -avoided crossing VB diagram (b) in conjugated //-electron—//-center species. Note that in the narrow range of the permitted distortion, d di, the vertical resonance energy is...

The corresponding radicals with 47V + 1 and/or 47V + 3 electrons possess also a degenerate electronic state with a zero vertical resonance energy.113166 In fact, the odd-nonalternant rings with 47V, 47V+ 1, and 47V + 3 electrons are truly antiaromatic , even though not all of them are referred to as such. We shall not dwell on these cases anymore, and hereafter we shall focus on n-electron—//-center systems (/ = 47V, 47V + 2 electrons). 

B. Thermochemical Properties of Delocalized Species Their Relation to Vertical Resonance Energies and jr-Distortivity... 

Benzene and other aromatics alike are stable molecules, while cyclobutadiene and other antiaromatic molecules are unstable molecules.27-76 Similarly, allylic species are stable intermediates and possess significant rotational barriers. It may appear as a contradiction that, for example, the tr-component of benzene can be distortive but it still endows the molecule with special stability or that the distortive jr-component of allyl radical can lead to a rotational barrier. We would like to show in this section that these stability patterns derive from the vertical resonance energy which is expressed as a special stability because for most experimental probes (in eluding substitution reactions) the o-frame restricts the molecule to small distortion167 in which the vertical resonance energy is still appreciable, as shown schematically in Figure 5. 

Scheme 23. Vertical Resonance Energy of the Reference Structure, B(R), for Benzene and Its Relation to the Vertical Energy of Hexatriene, B(h)...

Figure 6. Hess cycle used to relate the Dewar resonance energy of benzene, DRE(b), to the vertical resonance energy, Bq. Vertical resonance energy values for benzene and distorted benzene (calculated in this study111) are shown below the cycle. All energies are given in kcal/mol.

Bo(a) value for the symmetric species. As concluded repeatedly, here too the cr-frame constrains the allylic species to perform the rotation in the region where the vertical resonance energy is significant. The... 

A simple consequence of the above is the large vertical resonance energy of the distorted benzene in Figure 6 and its aromatic magnetic behavior both computed and measured.32 169-171 In fact, as emphasized by Choi and Kertesz,87 even when the aromatic [nj-annulenes undergo bond alternating distortion, their magnetic properties are still those of an aro-... 

The energy difference between the two-structure calculation and a one-structure calculation at the same geometry with optimal orbitals but lacking resonance/7t-electron delocalisation gives the vertical resonance energy (VRE) [70]. 

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