Nature of the hydrogen bond: crystallographic versus theoretical description of the O-H...N(sp ²) hydrogen bond

In order to analyze the characteristics of OH...N(sp 2) hydrogen bonds a survey of R--OH...N(sp 2) intermolecular interactions in organic crystals has been performed using the Cambridge Structural Database. Two subfiles of data, one containing 304 hydroxyl groups and the other 120 water molecules as donors, were selected and subjected to statistical analysis. In both sets the highest concentration of hydrogenbond interactions occurs for almost linear arrangements. The strength of interactions involving R--OH groups depends on R, increasing as follows: C(sp 3) < N < C(sp2). To complete this study we have carried out SCF calculations on 72 pyridine-water complexes at the 3-21G level. The nature of the hydrogen-bond interaction was also investigated by means of a topological analysis of both the charge density and the Laplacian of the charge density. This analysis revealed not only that charge-transfer interactions are sizeable but that they bear a direct relationship to the stability of the complex. Therefore, either the value of Vgp at the points of maximum charge concentration corresponding to the N(sp 2) lone pair, or the charge density at the hydrogen-bond critical point can be used as a suitable index for investigation of the relative stability of these hydrogen-bonded complexes. Our results show that * Authors for correspondence.