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 and in theoretical calculations.
In general, hydrogen bonds to sulfur not
only are weaker than those to oxygen but also show a marked preference
for a more “perpendicular” direction of
approach to the donor atom. Ab initio calculations at
the MP2/6-311++G(d,p) level on the complexes of
hydrogen
fluoride with H2O, H2S, H2CO,
and H2CS reproduce these differences, as does a search
of structures in the Cambridge
Crystallographic Data Base. We show that the Laplacian of the
charge density ∇2ρ predicts a qualitatively
correct
structure for all the systems considered, but gives poor quantitative
predictions of hydrogen-bonding geometries.
An analysis based upon Bader's atoms-in-molecules theory
rationalizes the differences between sulfur and oxygen
hydrogen bonds. A treatment of the hydrogen bond which explicitly
considers the contributions of atomic multipoles
to the electrostatic energy has more success than ∇2ρ
in predicting H bond directionality. Hydrogen bond
formation
to oxygen is driven by charge−charge interactions, whereas with
sulfur the stabilization arises principally from the
interaction of the charge on the acidic hydrogen with the dipole and
quadrupoles of sulfur.