Indole, resonance energy

Resonance energies of ca. 90, 182 and 330 kJ moF have been estimated for pyrrole, indole and carbazole respectively by comparing their protonation constants with those for selected model compounds (72C1(L)335, 72TL5019). 

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

Similar resonance is shown by indole. I T J (resonance energy... 

It is easily seen that the 7r-stabilization brought about by the introduction of a nitrogen atom is of the order indole > isoindole > indolizine. A confirmation of this consideration may be taken from a comparison of the resonance energies of indole,-isoindole and indolizine (REPE values (72T3657)). 

A linear relationship between the ratio of the vicinal coupling constants and the resonance energy has been found, indicating that the electron delocalization of the indole 7r-system is more pronounced than that of isoindole and indolizine respectively (77TL535) (see Table 5 and Section 3.08.2.1 for theoretical considerations). 

ERE determinations of indole have given results ranging from 4 I S to 57.6 kcal mole-1.263 Dewar et aZ.10-12 have calculated Dewar resonance energies for various benzo-fused compounds and obtained the following orders of decreasing resonance energy benzo[6]thiophene, 24.8 kcal mole-1 indole, 23.8 kcal mole-1 benzo[6]furan, 20.3 kcal mole-1. However,... 

HMO calculations on cycl[3.2.2]azine (240)26-26 attributed substantial DE to the molecule and more recently Dewar and Trinajstic462 calculated a value for the Dewar resonance energy of 18.9 kcal mole-1. The aromaticity of 240 is reflected in the PMR spectrum101 which shows proton resonances in the region 7.20 to 7.86 ppm. Paudler and Shin463 examined an isomer of 240, viz., pyrrolo[3,2,1-/, i]indole (241), and from a comparison of the benzene-induced shifts of the proton resonances in both 241 and 7-methylindole suggested that there is no loss of aromaticity in 241 relative to the indole derivative. A further comparison of the proton chemical shifts in 241 and its dihydro derivative 242 led the authors to suggest that the former is the more aromatic. 

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