Resonance-induced hydrogen bonding at sulfur acceptors in R1R2C=C and R1CS2- 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 Angstrom in pure thiones (R-1 = R-2 Csp(3)) to 1.75 Angstrom in thioureido species (R-1 = R-2 = N) and in dithioates -CS2-. The frequency of hydrogen-bond formation at =S increases from 4.8% for C=S similar or equal to 1.63 Angstrom to more than 70% for C=S > 1.70 Angstrom 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 -CS2- accept hydrogen bonds. C=S ... H-N,O bonds are shown to be significantly weaker than their C=OH ... 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 accepters. The directional properties and hydrogen-bond coordination numbers of C=S and C=O accepters 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.