The van der Waals Radii Cut-Off Criterion Is Not Useful

Alternately, a hydrogen bond permitted the insertion of a hydrogen atom between two nonhydrogen atoms without extending their separation. The implication 

Type of bond XH---A Observed range of H - - A distances (A) Covalent R (neutron va Criteria for H-bonds based on sums of van der Waals radii Top line W +WA — R —h Bottom line WH + WA-0.3  

There are two objections to this procedure. One is fundamental, because the cut-off is inconsistent with theory. The other objection to the use of van der Waals criterium is that it can sometimes give rise to incorrect conclusions. 

The use of a van der Waals distance cut-off criterion carries the wrong implication that hydrogen bonds become van der Waals interactions at longer distances. 

A survey of the hydrogen-bond lengths observed by neutron diffraction in the amino acids [60], for example shows that they have distributions with minimum and maximum values which correspond to those expected from a typical bonding potential energy curve, as shown in Fig. 2.5. There is no evidence of a cut-off distance beyond which another type of cohesive force takes over. Instead, the number of H A distances steadily decreases as they exceed the most probable H A bond length. In practice, it is observed that in crystal structures the first neighbor 

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