Interaction geometries and energies of hydrogen bonds to C=O and C=S acceptors: a comparative study

The occurrence, geometries and energies of hydrogen bonds from N-H and O-H donors to the S acceptors of thiourea derivatives, thioamides and thiones are compared with data for their O analogues - ureas, amides and ketones. Geometrical data derived from the Cambridge Structural Database indicate that hydrogen bonds to the C=S acceptors are much weaker than those to their C=O counterparts: van der Waals normalized hydrogen bonds to O are shorter than those to S by similar to 0.25 angstrom. Further, the directionality of the approach of the hydrogen bond with respect to S, defined by the C=S center dot center dot center dot H angle, is in the range 102-109 degrees, much lower than the analogous C=O center dot center dot center dot H angle which lies in the range 127-140 degrees. Ab initio calculations using intermolecular perturbation theory show good agreement with the experimental results: the differences in hydrogen-bond directionality are closely reproduced, and the interaction energies of hydrogen bonds to S are consistently weaker than those to O, by similar to 12 kJ mol(-1), for each of the three compound classes. There are no CSD examples of hydrogen bonds to aliphatic thiones, (Csp(3))(2)C=S, consistent with the near-equality of the electronegativities of C and S. Thioureas and thioamides have electron-rich N substituents replacing the Csp(3) atoms. Electron delocalization involving C=S and the N lone pairs then induces a significant >C delta+=S delta- dipole, which enables the formation of the medium-strength C=S center dot center dot center dot H bonds observed in thioureas and thioamides.