A theoretical study of the protonation of methylindole derivatives |
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Authors: | J Cataln P Prez M Yez |
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Institution: | Departamento de Química Física y Química Cuántica, Centro Coordinado CSIC-UAM, Facultad de Ciencias C-XIV, Cantoblanco Madrid, 34), Spain |
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Abstract: | Ab initio calculations on indole and all its mono-substituted methyl derivatives, using an STO-3G minimal basis set, show that the most basic site is C3. Protonation at the nitrogen atom cannot compete with protonation at C3; and C2 is the less basic site in all cases. The basicity increases with methyl substitution, with the only exception of 3-methyl indole. A good linear correlation exists between calculated and corresponding thermodynamics pK values. 2-Aminoindole is a much stronger base than methylindoles and its high pK value can be explained by the strong interactions with the solvent through tautomeric forms which accumulate positive charge at the NH2 group. Intramolecular quenching of the fluorescence of some indole derivatives involves intramolecular proton transfer to C4 rather than C2. Reasons why ring nitrogens can behave as either π-acceptors or π-donors in this series are discussed. |
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