Understanding the Hydrogen Bond in Terms of the Location of the Bond Critical Point and the Geometry of the Lone Pairs

The experimental charge density method has been employed to provide a more detailed description of the hydrogen bond in terms of the location of the bond critical point and the geometry of the lone pair of electrons. On the basis of a study of 7 different O−H···O hydrogen-bonded systems with 19 hydrogen bonds covering a wide range of hydrogen bond distances and angles, it has been possible to arrive at a generalization of the topological descriptors. In all of the hydrogen bonds studied, the electron density at the bond critical point (BCP) and its Laplacian fall in the range of 0.03−0.39 eÅ-3 and 0.7−6.0 eÅ-5, respectively. The bond paths deviate, in some instances widely, from the H···O bond axis, and the resulting dCP values (vertical displacement of the bond critical point from the internuclear line) range from 0.036 to 0.418 Å. The origin of such high dCP values has been related to the constellation of the various interaction centersthe lone pairs and the atom cores of the donor and the acceptor oxygens and the hydrogen atom. This study provides a useful classification of the hydrogen bonds in terms of a new interaction line, Li-j, connecting the various centers i and j. A nearness parameter, dL, that represents the perpendicular distance of the critical point from the interaction line, Li-j, justifies the classification. The dL values are found to be much smaller than the corresponding dCP values.