Analysis of the electron density distribution in clusters composed of hydrogen fluoride, water, and ammonia molecules, especially within the hydrogen-bond domains, reveals the existence of both σ- and π-binding between molecules. The σ-kind density distribution determines the mutual orientation of molecules. A π-system may be delocalized conjugated, which provides additional stabilization of molecular clusters. In those clusters where the sequence of hydrogen bonds is not planar, a peculiar kind of π-conjugation exists. HF anion and H5O2 cation are characterized by quasi-triple bonds between the electronegative atoms. The most long-lived species stabilized by delocalized π-binding are rings and open or closed hoops composed of fused rings. It is conjugated π-system that determines cooperativity phenomenon.

27 pages, 12 figures