Hydrogen Bonds - the Case of π and σ-Electrons as Proton Acceptors
|Sławomir J. Grabowski|
University of Łódź, Department of Crystallography and Crystal Chemistry (UŁ), Pomorska 149/153, Łódź 90-236, Poland
|Different types of interactions are analyzed for which the classical Pauling definition of hydrogen bonding  is hardly accepted. These are X-H...π (especially C-H...π)  and X-H...σ interactions [3, 4]. It is often stated that such interactions are rather weak if compared with typical hydrogen bonds, there are only few, mostly theoretical exceptions. However the new type of interactions was investigated recently using ab initio calculations. These are π...H+...π bonded complexes where the proton is strongly connected with one of π-electron systems which may be treated as the proton donating species and where the second π-electron system performs the role of acceptor. Such systems are characterized by meaningful binding energies (for some of cases over 20 kcal/mol) and the covalent character of both H+...π contacts . There are the other unique interactions, π...H+...σ and σ...H+...σ where also the H+...σ interactions are rather strong. The mentioned above interactions often fulfill the criteria and definition rules of hydrogen bonding and may be classified as the latter type of interaction. Additionally, it seems they are important interactions in crystal engineering and numerous physical and biochemical processes.
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Presentation: invited lecture at 18th Conference on Physical Organic Chemistry, Plenary session, by Sławomir J. Grabowski
See On-line Journal of 18th Conference on Physical Organic Chemistry
Submitted: 2006-05-30 08:14 Revised: 2009-06-07 00:44