Solvation effects on cation-π interactions:: a test study involving the quaternary ammonium ion

Water solvation effects on theoretical binding energies of the tetramethylammonium cation with benzene, phenol and indole have been analyzed as a prototype of biological cation-pi interactions. Solvent effects were introduced in the quantum chemical computations either by considering molecules belonging to the first solvation of the tetramethylammonium or by a polarizable continuum model. Our results show that the calculated binding energies are reduced by about three quarters with respect to the corresponding gas-phase results, but the sequence benzene<phenol<indole is preserved, in accordance with the concept of cation-pi interactions. Similar results are obtained for the interaction of tetramethylammonium with the benzene-indole pair.