Critical evaluation of C-H•••X hydrogen bonding in the crystalline state

Contacts of the type C-H...X where X = F, Cl, Br, I, O, S, or N were analyzed statistically using structures available in the Cambridge Structural Database. For this analysis, the cone correction method was extended to an isotropic density correction. This method corrects for both angular and distance effects by normalizing isotropic contact densities from the experimental distributions. It was possible to show the widespread occurrence of C-H...X interactions in the crystalline state using this method. Distances of highest incidence, at which the largest number of contacts were sampled after normalization of isotropic distribution, were determined for each interaction type. These distances were compared to the classically accepted van der Waals cutoff radii, often used to distinguish between hydrogen-bonded interactions and van der Waals contacts. Generally, distances of highest incidence did not occur at the sum of van der Waals cutoff radii but occurred either below or above that distance depending on the specific interaction analyzed. This methodology, while of course not yielding quantitative data on hydrogen bond strength, allowed for a qualitative ordering of the interactions studied, in terms of expected interaction strengths.