To take a comprehensive look at intermolecular interactions and classify them and to give a modern definition of the hydrogen bond, taking in to 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 it 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 Gold book of IUPAC). Hydrogen bonding was inferred by the difference in physical properties between otherwise chemically similar systems such as 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 by crystal structure analysis and NMR methods. Traditionally, hydrogen bond acceptors interact through a lone pair or +A8A 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 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 — +A8M interactions where +A8M 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 to 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 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.
> October 2005 – The Task Group met in Pisa, on 5-9 September 2005 in the form of a workshop. Eleven out of the fourteen task group members participated in the meeting. All task group members and 10 invited speakers gave talks 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 had several rounds of discussion during the Workshop and an interim report is now being prepared.
Workshop details are available at the URL address:
> workshop report and list of participants (pdf file – 23KB)
> August 2006 – A core-group of E. Arunan, G. R. Desiraju, R. A. Klein and J. Sadlej will be meeting in Bangalore between 18 and 22 September 2006. A one-day discussion has been organized that will include talks by the core-group members and some outside experts. After the meeting, core-group will come up with the final report. All comments about the project may be sent to [email protected]
> Jan 2010 – The task group has submitted a manuscript to be published in Pure and Applied Chemistry. This manuscript summarizes our understanding of hydrogen bonding that has evolved over the last century. It provides a definition for a hydrogen bond that is general and accurate. The provisional recommendations are going through the due review process with IUPAC. The preprints of the manuscript and the draft recommendation are available from [email protected]
> October 2010 -After an extensive review process and based on the suggestions from IUPAC, the task group submitted a recommendation and a technicalÂ report. The recommendation provides a novel definition of the hydrogen bond and the technical report provides a detailed summary of hydrogen bond research over the past century and a rationale for the proposed definition. Both these are available in the web page on Provisional Recommendations. After the public review period and any revisions that are required, the will appear in the same issue of Pure and Applied Chemistry.
A draft of the provisional recommendations is submitted to public review comments until 31 March 2011 (> provisional recommendations at ../provisional/abstract11/arunan_310311.html)
> July 2011 – IUPAC Recommendations published in Pure Appl. Chem., 2011, Vol. 83, No. 8, pp. 1637-1641, doi:10.1351/PAC-REC-10-01-02, published online 2011-07-08 ; Technical Report published in Pure Appl. Chem., 2011, Vol. 83, No. 8, pp. 1619-1636, doi:10.1351/PAC-REP-10-01-01, published online 2011-07-08
page last modified 16 August 2011