Electron-donor–acceptor complex formation by compounds of biological interest. Part III. Indole complexes

A nuclear magnetic resonance method has been used to determine the association constants of indole, 2-methyl-3-methyl-, 7-methyl-, 3-ethyl-, 3-dimethylaminomethyl-, 3-diethylaminomethyl-indole, benzothiophen, thiophen and pyrrole with either, or both, the electron acceptors 1,4-dinitrobenzene, 1,3,5-trinitrobenzene, in chloroform solution. The chemical shifts of different protons in the indole change relative to one another in the presence of large excesses of 1,3,5-trinitrobenzene. A similar phenomenon is observed in the spectrum of naphthalene.The differences in association constant of the various donors with a given acceptor, and the changes in the chemical shifts of the different protons of the donor with respect to one another, are accounted for by differences in the contribution to the intermolecular π-π interaction by different parts of the donor molecule. In general, positions of highest electron-density make the greatest contribution to this interaction.