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Hydrogen bonding interactions between indole and benzenoid-pi-bases.
M A Muñoz1, R Ferrero, C Carmona
1Departamento de Química Física, Facultad de Farmacia, Universidad de Sevilla, 41012 Seville, Spain. asuncion@us.es
Spectrochimica Acta. Part A, Molecular and Biomolecular Spectroscopy
|December 13, 2003
Summary
This study reveals that indole forms 1:1 complexes with aromatic bases like benzene, with interactions strengthening as base electron density increases. Higher concentrations suggest additional interactions, forming 1:2 complexes.
Area of Science:
- Physical Chemistry
- Spectroscopy
- Molecular Interactions
Background:
- Non-covalent interactions are crucial in chemical and biological systems.
- Understanding pi-pi and hydrogen bonding interactions is key to molecular recognition.
Purpose of the Study:
- To investigate the NH-pi interactions between indole and various aromatic compounds.
- To determine the stoichiometry and binding characteristics of these complexes.
Main Methods:
- Fourier transform infrared (FTIR) spectroscopy was used to study indole solutions.
- The NH stretching band of indole was monitored to detect complex formation.
Main Results:
- 1:1 complexes of indole with benzene, naphthalene, phenanthrene, toluene, m-xylene, and mesitylene were identified.
- NH frequency shifts indicated that methylation enhances electron density and strengthens interactions.
- Association constants increased with the number of rings and methyl groups on the aromatic base.
- Evidence for 1:2 complex formation at higher base concentrations was observed.
Conclusions:
- Indole engages in NH-pi interactions with aromatic systems.
- The strength of these interactions is influenced by the electron density and structure of the aromatic base.
- Complex formation can extend beyond simple 1:1 stoichiometry, involving van der Waals forces.