Conjugation in hydrogen-bonded systems
  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 \sigma- and \pi-binding between molecules. The \sigma-kind density distribution determines the mutual orientation of molecules. A \pi-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 \pi-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 \pi-binding are rings and open or closed hoops composed of fused rings. It is conjugated \pi-system that determines cooperativity phenomenon. 
Yulia V. Novakovskaya
physics.chem-ph physics.atm-clus