A combined quantum mechanical and molecular mechanical Monte Carlo simulation method was used to determine the free energy of binding between tetramethylammonium ion (TMA+) and benzene in water. The computed free energy as a function of distance (the potential of mean force) has two minima that represent contact and solvent-separated complexes. These species are separated by a broad barrier of about 3 kJ/mol. The results are in good accord with experimental data and suggest that TMA+ binds to benzene more favorably than to chloride ion, with an association constant of about 0.8 M-1.
Bibliographical noteFunding Information:
The National Institutes of Health (to J. Gao) and the National Science Foundation (to A. Auerbach) provided support for this research. The calculations were done on IBM RS/6000 computers at the State University of New York at Buffalo.