198 Topics in Current Chemistry Editorial Board: A. de Meijere KN ...

18 J.P. Glusker
Fig. 9. Citric acid monohydrate [34]. This stereoview shows hydrogen bonding that is cooperative,
extending through the crystal in such a way that oxygen atoms both give and receive
hydrogen bonds
Fig. 10. Repulsions and attractions in the H 2O◊◊◊HF complex
Spectroscopic studies [1] have shown that the H 2O◊◊◊HF interaction is such
that the HF molecule points its hydrogen atom directly towards the oxygen
atom, and that the axis of the HF molecule lies in the direction expected for a
tetrahedral arrangement of nonbonding lone pairs of electrons on the oxygen
atom. This implies that the HF molecule is recognizing an area of high electron
density and orienting itself in order to minimize the electrostatic energy of the
system. However, as shown in Fig. 10, it is necessary to consider not only the
electron density on the lone pair of electrons on H 2O, but also the charges on any
other atoms in the immediate neighborhood. For example, a point positive
charge near a water molecule gives a potential energy curve with two minima,
symmetrically placed at about ± 30°. The repulsive portion of the potential energy
equation has lowered the height of the barrier and moved the minima to
smaller angles.
The directions from which hydrogen bonds [9] approach sp 2 - and sp 3 -hybridized
oxygen atoms in ethers, ketones, esters, and epoxides have been studied.