34 No. 1
The IUCr Satellite Workshop on “Categorizing Halogen Bonding and Other Noncovalent Interactions Involving Halogen Atoms” was held 20–21 August 2011 in beautiful Sigüenza, Spain, at the Hospederia Porta Coeli (a facility of Alcalà University). The workshop was organized over two full days with nine oral scientific sessions and one poster session covering all of the research fields in which halogen bonding and other noncovalent interactions involving halogen atoms play a fundamental role.
Project chairs Pierangelo Metrangolo and Giuseppe Resnati (Politecnico di Milano) warmly greeted many of the 60 participants on the evening of the 19th and a late supper was enjoyed. The 60 workshop participants represented over 20 countries from North America, Europe, as well as China, Japan, India, and South Africa. The workshop began with opening remarks by the project chairs, who reiterated the role of the workshop:
- to take a comprehensive look at intermolecular interactions involving halogens as electrophilic species and classify them
- to give a modern definition of halogen bonding (XB), which takes into account all current experimental and theoretical pieces of information on both gaseous and condensed halogen-bonded systems in chemical and biological environments
The scientific sessions consisted of three or four 25-minute presentations given by selected scientists working in the field of halogen bonding, followed by five minutes of questions and discussion. Each session was chaired by one of the scientists presenting at the workshop. The topics of the sessions were as follows:
- Theoretical Modelling, Experimental Techniques
- Crystal Engineering
- Donors & Acceptors
- Materials (two sessions)
- A Proposed Definition of Halogen Bonding
|It is well-known that the electron density around the halogen nucleus is highly anisotropic, for this reason halogen atoms can be involved in a wide variety of intermolecular interactions (Figure reproduced from Clark, T.; Hennemann, M.; Murray, J. S.; Politzer, P. Halogen bonding: the σ-hole. J. Mol. Model., 2007, 13, 291)
I found the meeting to be very interesting and stimulating, as it brought together a diverse group of scientists each with their own expertise and perspective on the topic of halogen bonding. For example, Anthony Legon (Bristol, UK) discussed the information available from rotational spectroscopy on halogen-bonded complexes in the gas phase, Marc Fourmigué (Rennes, France) presented results on halogen bonding in molecular conductors, and P. Shing Ho (Fort Collins, Colorado, USA) presented anisotropic models for biological halogen bonds with an emphasis on nucleic acids. My own presentation discussed our multinuclear solid-state magnetic resonance studies of haloanilinium halides as well as a series halogen-bonded thiocyanates and selenocyanates. Lee Brammer (Sheffield, UK) discussed the role of halogen bonds in inorganic chemistry, Pavel Hobza (Prague) provided a computational perspective on the role of halogen bonding in drug design, Gautam Desiraju (IIS Bangalore, India) provided novel examples of the role of halogen bonding in crystal engineering, and Christer Aakeröy (Kansas State, USA) presented an elegant experimental study of the balance between halogen and hydrogen bonds. One of the slides from Aakeröy’s presentation humorously reflected some of the previous presentations which suggested that, depending on one’s perspective, the halogen bond could be seen as having much in common with the hydrogen bond or, conversely, as being substantially different from the hydrogen bond. There were several more excellent presentations, but the general topics mentioned above illustrate the breadth of approaches to studying halogen bonding, as well as the variety of fields of application. The oral presentations were punctuated with lively question-and-answer sessions. The poster session, held on a very hot Saturday night in the adjacent “Doncel’s House,” was equally lively.
|The CNC-IUPAC Travel Awards
David Bryce’s participation in this IUPAC workshop was an integral part of his CNC-IUPAC Travel Award. The Canadian National Committee/IUPAC travel awards were established in 1982 to enable young Canadian scientists to present their research at IUPAC-sponsored conferences outside continental North America. Applicants should be within 10 years of having obtained their Ph.D. The awards are sponsored jointly by the Canadian National Committee, IUPAC’s company associates, and the Gendron Fund, which was established in memory of a noted Canadian chemist who organized a highly successful IUPAC congress in Vancouver, British Columbia in 1981. Although many IUPAC member countries have national committees that handle liaison with IUPAC, to the best of our knowledge, the Canadian National Committee is the only one that operates a travel awards program. See www.cnc-iupac.ca/
awards_e.html for more details and future announcements.
The final session of the workshop was a round-table discussion among all attendees of the definition of halogen bonding. This was chaired by Roberto Marquardt, secretary of the IUPAC Physical and Biophysical Chemistry Division. He addressed the audience and gave an overview of the IUPAC project structure and, in specific, he introduced the halogen bonding project (IUPAC project 2009-032-1-100). Marquardt highlighted some important statements from various speakers which had emerged during previous sessions in the workshop. Several possible experimental and theoretical criteria for establishing the existance of a halogen bond were mentioned (e.g., “a halogen bond is a Lewis-acid-Lewis base attractive contact where a halogen atom is the interacting moiety in the Lewis acid”; “charge transfer might be strongly related to the existence of halogen bonds”; “the bond path and the bond critical point must be observed in the electron density between the halogen and the donor attached to it”, “can halogen bonding be seen as a case of an unbalanced 3-centre/4-electron bond?”). My own work suggested that the magnetic shielding tensor is sensitive to the halogen bonding interaction. The session continued with a presentation by E. Arunan on the IUPAC definition of hydrogen bonding, showing several examples where hydrogen bonding has been studied using different experimental and computational techniques. Arunan introduced a comparison between the hydrogen bond and halogen bond, showing several similarities and differences.
Co-chairs Metrangolo and Resnati then gave a summary of past and future events and activities associated with the IUPAC halogen bonding project. They noted that a draft of a preliminary recommendation for the definition of the halogen bond is expected to be delivered shortly. Resnati then started the round-table discussion with a question concerning how detailed the definition for a halogen bond should be. After some discussion, the majority consensus seemed to be that a flexible, simple definition would be most suitable and useful, rather than a strict definition. P. Sing Ho pointed out that to be practical, the definition must be understandable to a wide audience of scientists. There was then some discussion of the relevance of various existing experimental and theoretical evidence in the formulation of a definition, and it was felt that a simple definition should not have to rely on particular pieces of evidence, given that there is large number of experimental observations concerning the halogen bond.
Various special and/or extreme cases pertaining to the halogen bond were then addressed, including the I3- anion, fluorine as a halogen bond donor, and halonium ions. There was good agreement in these three cases that they all qualify as falling under the definition of halogen bonding. There was some animated discussion concerning various different possible theoretical descriptions of the halogen bond (e.g., role of electrostatics, polarization, charge transfer, and dispersion), including the presentation of some new data by Kevin Riley (Prague) which intimated the important role of dispersion.
Finally, some more subtle points were also raised: e.g., the terminology of “halogen bonding” vs. “the halogen bond,” and the proper representation of a halogen bond when drawing a molecular structure. Robin Rogers (Alabama, USA) closed the session with a presentation related to the journal Crystal Growth and Design, which will publish a special virtual issue featuring papers submitted by the speakers at this workshop (see http://pubs.acs.org/page/
cgdefu/vi/index.html). At the conclusion of the meeting, members of the IUPAC working group started working on a draft of the definition of halogen bonding.
From my perspective, this interactive and productive meeting brought together a diverse group of scientists, each with their own point of view on halogen bonding. This diversity is essential to establishing a consensus that supports a practical and accurate IUPAC definition of the halogen bond.
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