The nature of hydrogen bonds and conjugation in H-bonded systems

Analysis of the electron density distributions in the spatial regions of hydrogen bonds reveals the existence of both sigma and pi binding. The sigma-type electron density distribution is shown to be determined by occupied orbitals of both bonding and antibonding character. The pi system can be conjugated and provides the predominant binding of molecules in nonplanar structures. The mutual orientation of the molecules involved in a hydrogen bond is found to be optimal when the X-H bond in the proton donor and the orbital of the acceptor, the main contributor to sigma binding, have a common axis of symmetry. The conjugated pi electron system in molecular clusters determines the so-called cooperativity that is manifested in the deviation of the properties of hydrogen-bonded systems from pairwise additivity.