The van der Waals criterion for hydrogen bonding

Existing neutron diffraction data for hydrogen-bonded solids have been studied in the light of the van der Waals criterion for hydrogen bonding. It was found that for the formation of A--H. . .B bonds the distance A. . .B should be less than the sum of the A-H covalent bond distance, the van der Waals radius of H and that of B. It has also been shown that with decrease in A. . . B distance, the A--H bond extends in a quantitative manner irrespective of what atoms A and B are. Particularly, for A-H. . .A bonds when the overlap of the van der Waals radii of two A atoms exceeds a certain high value, the A-H bond is extended so much that the H atom is placed at the midpoint between A and A resulting in a symmetrical hydrogen bond.