Resonance-Induced Hydrogen Bonding at Sulfur Acceptors in R₁R₂C=S and R₁CS^-₂ Systems

The hydrogen-bond acceptor ability of sulfur in C=S systems has been investigated using crystallographic data retrieved from the Cambridge Structural Database and via ab initio molecular orbital calculations. The R₁R₂C=S bond lengths span a wide range, from 1.58 Å in pure thiones (R₁ = R₂ = Csp³) to 1.75 Å in thioureido species (R₁ = R₂ = N) and in dithioates -CS^-₂. The frequency of hydrogen-bond formation at =S increases from 4.8% for C=S ≃ 1.63 Å to more than 70% for C=S > 1.70 Å in uncharged species. The effective electronegativity of S is increased by conjugative interactions between C=S and the lone pairs of one or more N substituents (R¹R₂): a clear example of resonance-induced hydrogen bonding. More than 80% of S in -CS^-₂ accept hydrogen bonds. C=S⋯H-N,O bonds are shown to be significantly weaker than their C=O⋯H-N,O analogues by (a) comparing mean S⋯H and O⋯H distances (taking account of the differing non-bonded sizes of S and O and using neutron-normalized H positions) and (b) comparing frequencies of hydrogen-bond formation in 'competitive' environments, i.e. in structures containing both C=S and C=O acceptors. The directional properties and hydrogen-bond coordination numbers of C=S and C=O acceptors have also been compared. There is evidence for lone-pair directionality in both systems, but =S is more likely (17% of cases) than =O (4%) to accept more than two hydrogen bonds. Ab initio calculations of residual atomic charges and electrostatic potentials reinforce the crystallographic observations.