What Shall We Call a Hydrogen Bond

The chemical functional groups that participate in hydrogen bonds in organic compounds are given in Table 11.1, with ability as donor or acceptor generally decreasing down the table. 

One of the principal criteria for the recognition of the hydrogen bond in the solid state has been one of geometry, in particular the X... A and H. .. A distances. These distances generally fall within a fairly well-defined range for the commonly occurring X, A combinations as reviewed by Jeffrey and Saenger [10] and listed in Table 11.2. 

A hydrogen bond exists between a functional group X-H and an atom or group 

This general definition provides useful flexibility. The principle evidence for hydrogen-bond formation is association or chelation, i.e. the occurrence of molecular aggregates. We shall see that not only do these molecular aggregates form, but they do so in recognizable, and often predictable patterns. 

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