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–H⋯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, λ 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 0.47×103 exp(−2.5×d H⋯A ).