Physics:Resonance-assisted hydrogen bond

The resonance assisted hydrogen bond (commonly abbreviated as RAHB) is a strong type of hydrogen bond.

Causes

It is caused by the π-delocalization that involves the hydrogen and cannot be properly described by the electrostatic model alone. This description of the hydrogen bond has been proposed to describe unusually short distances generally observed between O=C-OH∙∙∙ or ∙∙∙O=C-C=C-OH.