Hydrogen Bonds Involving Sulfur: New Insights from ab Initio Calculations and Gas Phase Laser Spectroscopy

The hydrogen bonds involving sulfur (sulfur center hydrogen bonds; SCHBs) are generally regarded as weak H-bonds in comparison with the conventional N–H· · · O, O–H· · · O, N–H· · · N and O–H· · · N H-bonds. One of the reasons being considered for this is the smaller electronegativity of S than O or N. However, recent high resolution laser spectroscopy in combination with quantum chemical calculations reveals that SCHBs can be as strong as conventional H-bonds. Surprisingly, in the case of methionine containing dipeptides the amide-N–H· · · S H-bonds are even stronger than amide-N–H· · · O = C H-bonds. Sulfur is not only a potential H-bond acceptor, but the S–H group is also a very good H-bond donor and capable of forming a variety of H-bonds. For example, the S–H· · · π H-bond between H2S and indole/benzene is found to be the strongest H-bond among O–H· · · π, O–H· · · π, and C–H· · · π H-bonds. In general the SCHBs are dispersive in nature. This chapter details about few SCHB systems, many more systems need to be studied extensively and carefully to unravel many facts and facets about SCHBs. The major challenge for the experimentalists is to accurately determine the intra- and intermolecular H-bond energies and for the theoreticians to propose a universal H-bond descriptor.