Separating isomers with electric fields

A new technique uses electrostatic fields at ultracold temperatures to isolate individual conformational isomers from a complex molecule. The method should allow scientists to conduct new experiments into the differences between conformers, say the researchers.

Many complex molecules exist as different conformational isomers that can interconvert by chemical bond rotations. Researchers had previously discovered that the different conformers of an isolated molecule have different dipole moments - that is, the property of having a positive or a negative charge. Now, scientists at the Fritz-Haber-Institut in Berlin, Germany, and at the University of Aarhus, Denmark, have taken advantage of this fact to isolate individual conformers of the 3-aminophenol molecule.

’In general, the technique allows the separation the most polar species in a molecular beam from anything that is unpolar,’ says Frank Filsinger, lead author of the paper. ’In particular, it allows the separation of the molecules [to] be investigated from the atomic carrier gas.’ 

The team fired a molecular beam of helium seeded with 3-aminophenol molecules into an electrostatic deflector. Because the initial beam’s temperature is ultracold at around 1 Kelvin, the cis and trans conformers of the 3-aminophenol cannot interconvert and are thus stabilised. By using inhomogeneous electric fields, the polar 3-aminophenol molecules are steered through different trajectories, with only one making it cleanly to the laser interaction region where conformers are detected. 

Filsinger suggests that the availability of clean samples that consist of only one conformer will ’enable new experiments that aim [for] direct structural information like x-ray diffraction on gas-phase molecules with upcoming free-electron lasers.’ He also thinks it will allow scientists to study differences between conformers in experiments where it is difficult to address the conformers individually.

’It is a remarkable tour-de-force experiment,’ comments Timothy Zwier who researches conformational isomers at Purdue University, US. ’The fact that this is even possible was not clear prior to this work. One could imagine many experiments that might follow; beyond the spectroscopic characterisation, one could carry out conformation-specific reactions or photochemistry, for instance.’

David Pratt, professor of reactive chemistry at the University of Pittsburgh, US, thinks the work is important too. ’In principle, if one can find a way to separate the differently shaped molecules, then one might be able to "control" their chemistry, organising things so that one "pile" of molecules reacts with another substance whereas another "pile" does not,’ says Pratt. ’So it is very exciting to know that experiments like this are possible.’  

James Urquhart