Theoretical study of binding of tetramethylammonium ion with aromatics

Ab initio computations including correlation have been performed in a comparative study of complexes of tetramethylammonium (TMA) with benzene, pyrrole, pyridine, and imidazole, using polarized Gaussian basis sets of different accuracies. With the best basis (optimized on molecular polarizabilities), the BSSE-corrected binding energies in the most stable complexes of these four ligands are 9.1, 10.7, 13.3, and 16.3 kcal/mol, respectively, with benzene and pyrrole binding in a plane perpendicular to the TMA axis, and pyridine and imidazole inserting their nitrogen lone pair essentially along the TMA axis. The characteristics of secondary sites of binding of benzene are also determined and the overall results are discussed in connection with the possible role of aromatic amino acids in proteins.