Probing the hydrogen bond through experimental charge densities

Experimental charge density analysis has been carried out by using low-temperature X-ray diffraction data on molecular crystals exhibiting a variety of hydrogen bond interactions. The various types of hydrogen-bonded interactions examined include O-H (. . .) O, N-H (. . .) O, C-(HO)-O-. . ., O-H N-. . . N-. . .-H (. . .) N, N-H (. . .) S and C-H (. . .) S bonds. Static deformation density maps reveal depletion of electron density in the hydrogen-acceptor (H (. . .)A) regions. While the total density and the Laplacian at the H (. . .)A critical points exhibit an exponential dependence on the H (. . .) A distance, the positive curvature of the density, lambda(3), closely follows the exponential relation with a considerably better correlation factor. The relation derived based on the data for 218 hydrogen bonds of various types is given by 0.47 x 10(3) exp(-2.5 x d(H) . . . A). (C) 2003 Elsevier B.V. All rights reserved.