Estimates of the ab initio limit for sulfur-pi 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 alpha-helices in proteins. We studied the H(2)S-benzene dimer as the simplest possible prototype of sulfur-pi 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-zeta basis set (aug-cc-pVQZ). The equilibrium intermonomer distance for the hydrogens-down C(2)(v) configuration is 3.8 A 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 H(2)O-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 H(2)S-benzene is best thought of as arising from a favorable electrostatic interaction between partially positive hydrogens in H(2)S with the negatively charged pi-cloud of the benzene.