| Abstract
| - We have determined the experimental pKasfor the pyridine protonation and pyrrole deprotonationequilibria of the four isomeric (α-, β-, γ-, and δ-) carbolines(pyridoindoles) and for the deprotonationof their N-pyrido methylated derivatives. HF 6-31+G*ab initio calculations have been carriedout to obtain theoretically the magnitudes of the prototropicequilibria in gas phase and in solution.A cavity model of solvation has been employed. To analyze theinfluence of annelation andtautomerism, prototropic equilibria of azaindole isomers andH-pyrido tautomers of carbolines,respectively, have also been theoretically studied. Equilibriainvolving tautomeric species are alwaysenergetically unfavored against those of the normal forms.Solvation differently affects the relativegas phase values among isomers within the series and between the sameisomers of related series.Solvent damps the magnitude of the gap among azaindole orcarboline isomers and can even reversethe acidity sequence. The acidity sequence of theoretical freeenergies for the pyridine nitrogendeprotonation (γ<β<δ<α) and pyrrole nitrogen deprotonation(δ<β<α<γ) processes of the carbolinenormal forms in solution at 298 K are in quite reasonable agreementwith those observedexperimentally.
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