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Vol. 27 No. 3
May-June 2005

The Project Place | Information about new, current, and complete IUPAC projects and related initiatives
See also www.iupac.org/projects

Categorizing Hydrogen Bonding and Other Intermolecular Interactions

This project aims to 1) take a comprehensive look at intermolecular interactions and classify them and 2) create a modern definition of the hydrogen bond, taking into account all current experimental and theoretical information, and including hydrogen bonded systems both in gaseous and condensed phases, as well as in chemical and biological systems.

Hydrogen bonding has fascinated chemists and biologists for several decades now and is central to chemistry and biology. The original definition of hydrogen bonding invoked two electronegative atoms (X and Y) interacting through a hydrogen atom as in X-H•••Y. Initially X and Y were found to be mostly N, O and F which led to the mentioning of these atoms as part of the definition of hydrogen bonds in various sources (including the IUPAC Gold Book). Hydrogen bonding was inferred by the difference in physical properties between otherwise chemically similar systems such as are found between H2O and H2S. However, now it is well known that both H2O and H2S form a hydrogen bonded (H2X)2 dimer in the gas phase. Spectroscopic red shift in XH stretching frequency was among the first experimental evidence used for inferring hydrogen bonds. Now there are several hydrogen bonded systems that appear to show blue shift in XH stretching frequency. More interestingly, these systems have CH as the hydrogen bond donors, which was against conventional wisdom. The CH•••O interactions have been well established now in organic and biological systems through crystalline structure analysis and NMR methods. Traditionally, hydrogen bond acceptors interact through a lone pair or p bonded pair electrons. However, optically active hydrogen bonded complexes involving radicals have been found in the atmosphere. Matrix experiments and theoretical studies have shown that a CH3 radical could form a complex with H2O, which could be represented as C•••HO. Are these one electron hydrogen bonds with C as the acceptor? There have been reports on X-H•••σ interactions where σ electrons act as hydrogen bond acceptors. Dihydrogen bonds have been observed in which H in XH (X=electronegative) interacts with another hydrogen in MH (typically a metal hydride) with partial negative charge. Moreover, there have been numerous reports on H2 molecular complexes in the literature—should these be regarded as containing hydrogen bonds?

Electrostatic interaction was identified as the dominant factor for hydrogen bonds. Recent NMR and Compton scattering experiments have given evidence for partial covalency in hydrogen bonds. Dispersion forces have been shown to dominate hydrogen bonded complexes of second row hydrides (HCl and H2S). Chlorine monofluoride (ClF) has been shown to form weakly bound complexes with bases very much like HF and these have been identified as chlorine bonded complexes. Such chlorine bonding interactions have been observed in crystal structures as well. Hydrogen bonding, electrostatic interactions, and van der Waals interactions are all loosely and interchangeably used in the field.

Often van der Waals forces are equated with dispersion forces, though the origin of van der Waals forces (from the equation named after him) should include all intermolecular forces. Should rare gas complexes such as Ar-Ne be called London molecules instead of van der Waals molecules, as only London dispersion forces contribute to the stabilization of Ar-Ne? Should Ar-HF be called hydrogen bonded or van der Waals complex? This project will attempt to give a modern definition of a hydrogen bond that is as inclusive as possible. Also, intermolecular interactions will be categorized logically considering the physical forces involved.

The task group is chaired by Elangannan Arunan (Bangalore, India) and Steve Scheiner (Utah, USA). Other members are Ibon Alkorta (Madrid, Spain), David C. Clary (Oxford, UK), Robert H. Crabtree (New Haven, USA), Joseph J. Dannenberg (NY, NY, USA), Gautam R. Desiraju (Hyderabad, India), Henrik G. Kjaergaard (Otago, New Zealand), Roger A. Klein (Bonn, Germany), Karl Kleinermanns (Düsseldorf, Germany), Anthony C. Legon (Exeter, UK), Benedetta Mennucci (Pisa, Italy), David J. Nesbitt, (Colorado, USA) and Joanna Sadlej (Warsaw, Poland).

The Task Group will hold a workshop in Pisa from 5–9 September 2005. All the participating members of the Task Group will be presenting a summary of their recent work in the area of hydrogen bonding and molecular interactions and will also give their views about the classification of inter- and intra-molecular interactions. The Task Group will hold several rounds of discussion during the workshop. It is intended that a provisional position paper will be produced at its conclusion. There will also be sufficient room for an additional 25–30 non-task group participants to take part in the workshop. Anyone interested in participating should contact either Elangannan Arunan <arunan@ipc.iisc.ernet.in> or Roger Klein <klein@institut.physiochem.uni-bonn.de>.

For more information, contact the Task Group Chairman Elangannan Arunan <arunan@ipc.iisc.ernet.in>. Comments and suggestions from IUPAC members or anyone else interested in the project are welcome.

www.iupac.org/projects/2004/2004-026-2-100.html


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