DIRECTIONALITY OF HYDROGEN-BONDS TO SULFUR AND OXYGEN

Hydrogen-bonded complexes involving sulfur bases are found to be quite different from the analogous oxygen complexes, both experimentally an d in theoretical calculations. In general, hydrogen bonds to sulfur no t only are weaker than those to oxygen but also show a marked preferen ce for a more ''perpendicular'' direction of approach to the donor ato m. Ab initio calculations at the MP2/6-311++G(d,p) level on the comple xes of hydrogen fluoride with H2O, H2S, H2CO, and H2CS reproduce these differences, as does a search of structures in the Cambridge Crystall ographic Data Base. We show that the Laplacian of the charge density d el(2) rho predicts a qualitatively correct structure for all the syste ms considered, but gives poor quantitative predictions of hydrogen-bon ding geometries. An analysis based upon Bader's atoms-in-molecules the ory rationalizes the differences between sulfur and oxygen hydrogen bo nds. A treatment of the hydrogen bond which explicitly considers the c ontributions of atomic multipoles to the electrostatic energy has more success than del(2) rho in predicting H bond directionality. Hydrogen bond formation to oxygen is driven by charge-charge interactions, whe reas with sulfur the stabilization arises principally from the interac tion of the charge on the acidic hydrogen with the dipole and quadrupo les of sulfur.