Tetrahydrofuran (THF) is a small ring-shaped molecule consisting of four carbon atoms and an oxygen atom, with two hydrogen atoms attached to each carbon. It is a favourite of chemists who use it as a solvent to carry out both chemical and biochemical reactions important to the pharmaceutical industry, for example. THF dissolves a wide range of compounds; it also remains a liquid at quite low temperatures and is easily evaporated, offering a medium that is readily separated from reaction products. Millions of litres are used each year by the magnetic tape industry.
Despite its simple chemical structure and widespread use, little has been known about how the molecules arrange themselves in the liquid; the only work done has been theoretical. And yet understanding solvent structure and the resulting interactions with dissolved materials is extremely important in optimising chemical reactions and processes. It is particularly relevant in the growing area of nanoscience, whereby liquids with a complex nanostructure are being designed to have highly controlled properties.
With this in mind, Daniel Bowron at ISIS and collaborators at University College London have been using the ISIS SANDALS instrument, which is specially designed for such studies, to examine the three-dimensional structure of liquid THF. The technique has the advantage that substituting the hydrogen in THF with deuterium can give the required information. This is because the heavier isotope has a different scattering strength for neutrons and so gives different results. By comparing the neutron scattering patterns of the deuterated and non-deuterated solvent, plus mixtures containing proportions of both, the researchers were able to work out the relative molecular orientations. 

Swiss cheese structure 

The neutron data showed that the molecular rings assembled in T-like arrangements as a result of electrostatic attraction between the oxygen atoms and the carbon-hydrogen units of neighbouring molecules. The packing created voids with a slight positive charge. This latter characteristic also explained THF’s useful ability to trap free electrons. Such solvated electrons are of enormous interest as they are a useful probe in understanding key chemical and biological reactions that involve the transfer of electrons.
The researchers went on to study the effects of adding a small amount of water, finding that the interactions in both types of molecule changed the structure of the mixture in a complex way. In particular, the slight negative charge on oxygen atoms in water tended to offset the positive charge of the THF cavities – an important consideration in electron-transfer reactions.

Dr Daniel Bowron (ISIS), Alan Soper (ISIS), John Finney (University College London)

Research date: December 2006