Resonance-Induced Hydrogen Bonding at Sulfur Acceptors in R ₁ R ₂C=S and R ₁CS₂ ⁻ 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 R 1 R 2 C=S bond lengths span a wide range, from 1.58 Å in pure thiones ( R 1 = R 2 = C sp 3 ) to 1.75 Å in thioureido species ( R 1 = R 2 = 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 ( R 1 R 2 ): 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.