Sulfur, not too far behind O, N, and C: SH...pi hydrogen bond

We report hydrogen-bonded complexes of H(2)S with indole and 3-methyl indole stabilized by the S-H...pi interaction. It is interesting to discover that although sulfur and its hydrides are known as poor hydrogen-bond donor/acceptors, sulfur is not too far behind oxygen, nitrogen, and carbon in regard to forming the pi-type hydrogen bonds. This report also extends the scope of our earlier studies from sigma-type hydrogen-bonded complexes of sulfur (O-H...S and N-H...S sigma-type hydrogen-bonded complexes) to pi-type hydrogen-bonded complexes of sulfur (S-H...pi pi-type hydrogen-bonded complexes). The experiments were carried out using the supersonic jet expansion technique, and the complexes were probed using laser-induced spectroscopy such as laser-induced fluorescence (LIF), resonant two-photon inonization (R2PI), and fluorescence dip infrared spectroscopy (FDIRS). The FDIR spectroscopy revealed that while there was no shift in the N-H stretch, the S-H stretch was red shifted by about 21 cm(-1). For the H(2)O complexes of indole and 3-methylindole, however, there was a significant red shift in the N-H stretch. These observations suggest that H(2)O forms a NH...O type complex, whereas H(2)S prefers to form a SH...pi type complex. The experimental results were complemented by ab initio calculations and energy decomposition analysis. The binding energies for both the sigma-type and pi-type hydrogen-bonded M.L complexes (M = indole and 3-methylindole; L = H(2)O and H(2)S) were calculated by extrapolating MP2 interaction energies to the complete basis set limit. The calculated M.H(2)S (sigma-type) interaction energy (2.74 kcal/mol) was considerably smaller than that of the M.H(2)S pi-type hydrogen-bonded complex (4.89 kcal/mol), which is exactly opposite of the trend found for the M.H(2)O complexes. This is consistent with the experimental observations. Comparison of the S-H...pi interaction with the other type of X-H...pi (X = C, N, and O) shows that the S-H...pi interaction is the strongest among them. In all of the pi-type HB complexes, the dispersion energy component has significant contribution to the total binding energy.