The Importance of a and n Cooperativity

The effect on the covalent O-H bond length is so small that it has not been observed experimentally. 

In this case, the lengthening of the N-H and C=0 bonds, and the shortening of the C—N bonds as a consequence of hydrogen-bond formation can be measured by careful X-ray or neutron diffraction analyses, as described in Chapter 5. This type of -cooperativity is especially important in the main-chain hydrogen bonding of proteins, where it increases the double-bond character and hence the torsional rigidity of the peptide C-N bond. 

7r-Bond cooperativity also contributes to the stabilization of the Watson-Crick base pairs in the nucleic acids by electron delocalizations shown in Fig. 2.7. In the enol form of some /7-diketone moieties, it is referred to as resonance-assisted hydrogen bonding [108a]. 

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