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, λ3, 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×dH···A)