Acid-Base Formalism in Dispersion-Stabilized S-H•••Y (Y=O,S) Hydrogen-Bonding Interactions

The role of sulfhydryl (S-H) group as hydrogen bond donor is not as well studied as that of hydroxyl (O-H). In this work we report on the hydrogen-bonding properties of S-H donor in 1:1 complexes of H2S with diethyl ether (Et2O), dibutyl ether (Bu2O), and 1,4-dioxane (DO). The complexes were prepared in supersonic jet and investigated using infrared predissociation spectroscopy based on VUV photoionization detection. The IR spectra of all the complexes showed the presence of a broad, intensity-enhanced, and red-shifted hydrogen-bonded S-H stretching transition. The S-H stretching frequency was red-shifted by 46, 63, and 49 cm(-1) in H2S-Et2O, H2S-Bu2O, and (HS)-S-2-DO complexes, respectively, suggesting that all the complexes are S-H center dot center dot center dot O bound. Computationally, two different S-H center dot center dot center dot O bound structures, namely, "coplanar" and "perpendicular", were obtained as the minimum energy structures for these complexes at the MP2/6-311++G** level, with the former being the global minimum. However, with Dunning-type basis sets (aug-cc-pVDZ and aug-cc-pVTZ) only the perpendicular structures were found to be stable at the MP2 level. The large widths of the bound S-H stretch observed in the experimental spectra (fwhm of 35 to 80 cm(-1)) were attributed to inhomogeneous broadening due to multiple conformations of the alkyl chains in the coplanar and perpendicular structures populated in the jet. The frequency shifts in the hydrogen-bonded S-H stretching mode as well as the bond dissociation energies of all S-H center dot center dot center dot Y (Y=O,S) complexes of (HS)-S-2, which includes the (HS)-S-2 dimer and H2S-methanol (H2S-MeOH) complexes reported in our previous work (ChemPhysChem 2013, 14, 905-914), were found to scale linearly with the proton affinity of the acceptor molecule. In this regard the S-H group, like O-H, is found to conform to the widely accepted acid-base nature of hydrogen-bonding interactions.