Fundamental Properties of Hydrogen Bonds

Despite its ubiquity, it is not necessarily straightforward to define the H bond without ambiguity. A H bond can be generally described as 

In view of the complexity and diversity of H bonds, these rather simple definitions are convenient and safely encompass many different types of H bonds. Many exceptions may arise when one attempts to frame a universal, or specific, definition of the H bond (e.g., how to choose the distance or angle cutoff). Nonetheless, detailed H-bond criteria may be helpful because the boundary between the H bond and many other types of interactions, for example, the covalent bond, the ionic interaction, the van der Waals (vdW) interaction, and so on, is sometimes indistinct Thus, investigation of H bonds from a more pragmatic perspective may require many detailed criteria that allow us to discern whether a given bond is a H bond or not. In fact, a few operational criteria have recently been proposed that would help characterize many types of H bonds [9]. 

Infrared (IR) and Raman spectroscopy techniques are particularly useful for the characterization of H bonds [1]. These techniques exploit the fact that formation of a H bond results in a shift of the X- H stretching vibrational mode to a lower frequency (redshift), often by several hundred per centimeter. Moreover, the X-H 

Stretching peak is broadened and intensified upon formation of a H bond. NMR spectroscopy can also be used to detect H bonds because formation ofa H bond leads to a downfield chemical shift. X-ray diffraction analysis and neutron diffraction analysis allow us to obtain structural details of H bonds. Determination of the position of the hydrogen atom in small molecules is possible with these methods, although the r(X-H) values determined by X-ray analysis tend to be shorter than the real internuclear distances [lej. However, for larger biomacromolecules such as proteins, the positions of hydrogen atoms are usually not resolved by X-ray diffraction [16]. 

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