C-H hydrogen bonding: 3. Hammett correlations for phenylacetonitriles and tetrafluorobenzenes with HMPA

Understanding the hydrogen bond, or H-bond, arguably the strongest known intermolecular attractive force, is of great importance. Models of H-bonding hold that both electrostatic and covalent components are at work. H-bonding of C-H bonds, and thus presumably other A-H bonds, is greatly facilitated by polar substituents. We have found that equilibrium constants, K, for C-H H-bonding in series of phenylacetonitriles and tetrafluorobenzenes with HMPA are closely correlated by the Hammett equation. That is, K values vary as substituents of different polarities are introduced at positions remote from the donor C-H bond. This result is consistent with a significant electrostatic component in these examples of H-bonding. It may, however, also be explained by interaction with the sigma* orbital of the C-H bond, which is involved in the covalent component of the H-bond. Theoretical models need to be applied to this and other cases. Copyright (C) 2010 John Wiley & Sons, Ltd.