Estimates of the ab Initio Limit for Sulfur−π Interactions: The H₂S−Benzene Dimer

The interaction between aromatic rings and sulfur atoms in the side chains of amino acids is a factor in the formation and stabilization of α-helices in proteins. We studied the H2S−benzene dimer as the simplest possible prototype of sulfur−π interactions. High-quality potential energy curves were obtained using coupled-cluster theory with single, double, and perturbative triple substitutions (CCSD(T)) and a large, augmented quadruple-ζ basis set (aug-cc-pVQZ). The equilibrium intermonomer distance for the hydrogens-down C2v configuration is 3.8 Å with an interaction energy of −2.74 kcal mol-1. Extrapolating the binding energy to the complete basis set limit gives −2.81 kcal mol-1. This binding energy is comparable to that of H2O−benzene or of the benzene dimer, and the equilibrium distance is in close agreement with experiment. Other orientations of the dimer were also considered at less complete levels of theory. A considerable reduction in binding for the sulfur-down configuration, together with an energy decomposition analysis, indicates that the attraction in H2S−benzene is best thought of as arising from a favorable electrostatic interaction between partially positive hydrogens in H2S with the negatively charged π-cloud of the benzene.