Number: 2004-026-2-100
Title: Categorizing hydrogen bonding and other
intermolecular interactions
Task Group
Chairmen: Elangannan
Arunan and Steve
Scheiner
Members: Ibon
Alkorta, David C. Clary,
Robert H. Crabtree,
Joseph J. Dannenberg,
Gautam R. Desiraju,
Pavel Hobza, Henrik
G. Kjaergaard, Roger
A. Klein, Karl
Kleinermanns, Anthony
C. Legon, Benedetta
Mennucci, David J.
Nesbitt, and Joanna
Sadlej
Objective:
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.
Description:
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 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 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 --- s
interactions where s 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.
Progress:
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:
http://institut.physiochem.uni-bonn.de/IUPAC_Pisa2005/Workshop.html
> workshop
report and list of participants (pdf file - 23KB)
Aug 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]
The details of the one-day discussion meeting are
available at: http://ipc.iisc.ernet.in/~arunan/iupac.html
All of the talks are available at http://ipc.iisc.ernet.in/~arunan/iupac.
> Report/update published in Chem.
Int.
Mar-Apr 2007, p. 16
Last update: 16 April 2007
<project announcement published
in Chem.
Int.
May/Jun 2005>