π – π interactions in aromatic liquids

T- and Y- shaped benzene

A slight shift in molecule orientation affects how the benzene molecules interact chemically with other molecules such as proteins. The ‘T-shaped’ arrangement has a single hydrogen atom pointing into the centre of the pair benzene molecule. The ‘Y-shaped’ arrangement is slightly rotated so that two hydrogen atoms are pointing to the other molecule’s delocalised electron ring.
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Aromatic π – π interactions between benzene-like chemical groups play a key role in many important scientific phenomena, including the stereochemistry of organic reactions, organic host-guest chemistry and crystal packing, protein folding and structure, DNA and RNA base stacking, protein-nucleic acid recognition, drug design, and asphaltene (heavy crude oil) aggregation and fouling.

We have used isotopic labelling and neutron diffraction to measure the structure in two archetypal aromatic liquids: benzene and toluene. Our results call for a significant rethink of the way we model π-orbital interactions. We have found that the so-called ‘T-shaped’ arrangement between aromatic groups, which had previously been proposed as a dominant motif in π – π interactions, is in fact absent in our liquids. Instead, we have observed an entirely new geometry, which we have christened ‘Y-shaped’.In this configuration molecules direct hydrogen atoms towards the π-orbitals themselves, rather than towards the centre of the aromatic ring.

TF Headen, CA Howard, NT Skipper, MA Wilkinson (University College London), DT Bowron, AK Soper (ISIS)

Research date: January 2008

Further Information

Contact: CA Howard, c.howard@ucl.ac.uk

Further reading: TF Headen et al., J Am Chem Soc 132 (2010) 5735

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