Figure 1.

Energy density distribution H (r) (blue solid lines for negative values; red dashes for positive values) for two glycine zwitterions approaching to form a strong hydrogen bond. The plots are drawn in the plane containing the carboxylic group of the HB acceptor molecule: top, long distance between donor and acceptor atoms (8 Å); center the equilibrium position in the crystal (2.76 Å); bottom, very short distance as in symmetric hydrogen bonds (2.4 Å). Note that H(r) has uninterrupted regions of negative values in the bottom plot. A weaker C − H^…O bond path is also calculated (dashed bond path).