IUPAC set to redefine hydrogen bonding following the initiatives

'Hydrogen bonding is not string theory or gravitons, which no-one has ever seen yet. There are plenty of experimental and theoretical results available.'

       ~E. Arunan as quoted in Chemistry World, 3 Novemberr 2010

          There is more than one paper published every hour on hydrogen bonding. Still most authors, including experts, feel uncertain about what a hydrogen bond is. Prof. E. Arunan from the Inorganic and Physical Chemistry contacted IUPAC in 2004 and suggested that the international body should come out with a modern definition of hydrogen bonding. He has been working on H2O and H2S complexes and found them to have similar 'hydrogen bonded' geometry though in bulk H2O and H2S are like 'apples and oranges'. His group has also defined 'hydrogen bond radii' for the first time and showed the correlation with the dipole moment of hydrogen bond donors.

          IUPAC in turn asked him to form a task group of international experts and come up with a definition. Apparently, an earlier attempt through IUPAC did not succeed. Arunan contacted 13 other experts including Prof. Gautam R. Desiraju who was at the University of Hyderabad then. He had authored a popular book on 'Weak Hydrogen Bond' and made voluminous contributions to the field. Prof. Desiraju has since moved to IISc and is now a Professor at the Solid State and Structural Chemistry Unit. Prof. Steve Scheiner, author of another popular book on hydrogen bonding felt it is an important task and became part of a core group. Dr. Roger A Klein from University of Bonn, an expert on using electron density topology to study hydrogen bonding and Prof. Joanna Sadlej, a theoretical chemist/spectroscopist from University of Warsaw completed the core group. In addition the task group included the following: Prof. Robert Crabtree from Yale who observed the new phenomenon of dihydrogen bonding. Prof. A. C. Legon from Bristol who has done extensive work on halogen bonding and hydrogen bonding. Prof. Pavel Hobza from the Academy of Sciences of Czech Republic, who had predicted blue-shifting hydrogen bonding theoretically, challenging the red-shift = hydrogen bond concept., Prof. David Nesbitt, University of Colorado, an expert in experimental spectroscopy and dynamics who has worked on the elusive protonated methane and complexes between rare gas atoms and HF. Prof. David Clary, Oxford University, an eminent theoretician who has worked on rare gas-HF and benzene-H2O complexes. Prof. Henrik G. Kjaergaard, University of Copenhagen, who has used overtone spectroscopy as a tool to look for hydrogen bonding. Prof. Benedetta Mennucci, University of Pisa who is an expert on condensed phase modeling and also an Author of the popular Gaussian code. Prof. Joseph J. Dannenberg, City University of New York, who has done extensive work on covalency in hydrogen bonding challenging the conventional electrostatic model. He was also asked to be part of an earlier IUPAC attempt at defining hydrogen bond. Prof. Ibon Alkorta, Instituto de Quimica Medica, Madrid, Spain, an expert in electronic structure theory and electron density topology who has shown that one electron is enough to form a hydrogen bond. The task group had two meetings and extensive email discussions.

          After five years, it submitted a report to IUPAC which included the definition of hydrogen bond in early 2010. Following an extensive review process (25 reviews as opposed to typical 2 reviewers for a manuscript), the task group has recently submitted
1) A recommendation containing the definition of the hydrogen bond
2) A technical report that summarizes hydrogen bond research over a century and provides a rationale for the proposed definition. These have appeared in the IUPAC website as provisional recommendation and are available at the following link.
http://www.iupac.org/web/nt/2010-10-25_hydrogen_bond
Chemistry World, Royal Society of Chemistry, news magazine has featured a story on this important project and it may be read online:
http://www.rsc.org/chemistryworld/News/2010/November/03111001.asp