Hydrogen-bond acceptor and donor properties of divalent sulfur (Y-S-Z and R-S-H)

The hydrogen-bond acceptor ability of divalent sulfur in Y-S-Z systems, Y, Z = C, N, O or S, and the donor ability of thiol S-H have been studied using crystallographic data retrieved from the Cambridge Structural Database. Of 1811 Y-S-Z substructures that co-occur with N-H or O-H donors, only 86 (4.75%) form S ... H-N,O bonds within S ... H < 2.9 Angstrom. In dialkylthioethers, the frequency of S ... H bond formation is 6.24%, but drops below 3% when the alkyl groups are successively replaced by Csp(2) centres. This parallels an increasing delta-positivity of S as calculated using ab initio methods. A similar frequency trend is observed for O ... H-N,O bond formation by analogous oxyethers. Mean intermolecular >S ... H distances for O-H [2.67(3) Angstrom] and N-H [2.75 (2) Angstrom] donors (with H positions normalized to neutron values) are ca 0.25 Angstrom longer than in C=S ... H-N,O systems, indicative of very weak hydrogen bonding to >S. Intramolecular >S ... H are slightly more frequent (8.56%), with S ... H slightly shorter than for the intermolecular case. In contrast, 26 (70.3%) out of 37 S-H donors that co-occur with suitable accepters form X ... H-S bonds. The C=O ... H-S system is predominant with a mean O ... H distance of 2.34 (4) Angstrom, considerably longer (weaker) than in C=O ... H-O systems.