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 σ and π binding. The σ-type electron density distribution is shown to be determined by occupied orbitals of both bonding and antibonding character. The π 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 σ binding, have a common axis of symmetry. The conjugated π 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.
Keywordshydrogen bond conjugation cooperativity electron density distribution
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- 7.R. F. W. Bader, Atoms in Molecules: A Quantum Theory (Clarendon Press, Oxford, 1990).Google Scholar