Strength of hydrogen bonds and theoretical description

Measurement of the properties of H-bonded systems over a range of temperatures leads to experimental values of AG, AH and AS for H-bond formation, and these data have been supplemented in recent years by increasingly reliable ab initio quantum-mechanical calculations. Some typical values for the enthalpy of dissociation of H-bonded pairs in the gas phase are in Table 3.9. 

It follows from the preceding discussion that the unbranched H bond can be regarded as a 3-centre 4-electron bond A-H B in which the 2 pairs of electrons involved are the bond pair in A-H and the lone pair on B. The degree of charge separation on bond formation will depend on the nature of the proton-donor group AH and the Lewis base B. The relation between this 3-centre bond formalism and the 3-centre bond descriptions frequently used for boranes, polyhalides and compounds of xenon is particularly instructive and is elaborated in 

It follows from the preceding discussion that the unbranched H bond can be regarded as a 

3-centre bond descriptions frequently used for boranes, polyhalides and compounds of xenon is particularly instructive and is elaborated in 

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