Charge Density in H-bonds and Other Weak Interactions. Bryan Craven, Dept. of Crystallography, University of Pittsburgh, Pittsburgh, PA 15260.

Crystal stucture determinations can yield much more than interatomic distances and angles when details of the electronic charge distribution are considered. For example a typical electrostatic potential for a molecule isolated from the crystal indicates the lack of directionality to be expected for X-H...Y angles in hydrogen bonds. Also, such maps seldom show effects from "lone pairs" at Y, indicating that these are not important in governing the geometry of electrostatic interactions at Y. The broad features in the potential and its long range 1/r dependence make it difficult to set up distance and angle criteria for determining whether or not a given interaction should be classed as a hydrogen bond.

In the X-H group, there is always a local dipole (about 1 Debye) at the H-atom because, with respect to the nucleus of an isolated spherical H, charge density is displaced into the covalent bond leaving a deficiency in charge density on the opposite side. This is irrespective of the electronegativity of X. Weak molecular interactions will occur involving the H-dipole even when X is a C-atom. When X is electronegative (O or N), the spherical part of the charge density around H is depleted by about 0.2e (relative to X=C). This second effect will further enhance the interaction with an electronegative neighbor Y and thus lead to a conventional hydrogen bond, X-H...Y.

Examples will be drawn from experimental charge density studies that involve O-H...O, N-H...N, C-H...O and C-H...C=C interactions.

This work was supported by a grant GM-39513 from the NIH.