H-bond Geometries

It is the aim of this chapter (1) to discuss the basic experiments for the detection of trans H-bond couplings and to indicate experimental problems, (2) to briefly describe the range of groups for which such couplings have been observed and to summarize their sizes, and (3) to present a number of recent applications which characterize changes in H-bond geometries of biomacromolecules. 

Table 3. H-bonding geometry for diphenylguanidinium nicotinate hydrate (A,").

The components of the SCF interaction energy, e ch, eU0) and AE F, are shown in Fig.6. It is clear that the anisotropy is determined by the electrostatic contribution. This term generates both a minimum for the H-bond geometry and the barriers for the H-to-H and 0-to-0 configurations. 

Calculations lead to a similar conclusion for the FIF -I- CI2 pair both of these structures have been subsequently observed by their infrared spectra in solid Ar and Ne". Due to the questionable existence of the H-bonded geometry, as well as the very weak interaction energy of only 2 kcal/mol or less, one can conclude that the dihalogen molecule does not act as a proton acceptor in a H-bond. 

One important facet of a hydrogen bond is its equilibrium structure. There are several layers in the hierarchy of accuracy of prediction of H-bond geometries. Work over the last several years has shown that one can make a very educated guess as to the general orientation of the two molecules based on simple considerations that do not involve much computational expense. 

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