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August 27, 2010

Taking aim at a slippery target

Powerful synchrotron light captures never-before-seen electronic interactions of molecules dissolved in liquids

Figure 1: Acetic acid molecules, best known as the main component of vinegar, can change electronically when mixed into a solvent.

© 2010 iStockphoto/ matka_Wariatka

When molecules take the plunge into a liquid solvent, they undergo constant twists and turns as they spread out and interact within their new environment. Such haphazard movements in solvents make it difficult for scientists to measure specific reactivity changes. “Especially for liquids and solutions, it is not known how intermolecular interactions affect the electronic structure of the reactants, despite this being one of the key physical processes of chemistry,” says Takashi Tokushima from the RIKEN SPring-8 Center in Harima. 

Now, Tokushima and colleagues from RIKEN and Hiroshima University have used high-energy synchrotron light to capture the signals of molecular orbitals (MOs)—quantized spatial distributions of electrons that determine chemical reactivity—from acetic acid molecules in solution1 (Fig. 1). This approach enables the measurement of solvation effects with atom-by-atom precision, which is crucial information for understanding essential reactions such as enzyme-based catalysis.

The researchers achieved their result by smashing accelerated photons into an acetic acid solution, setting off an x-ray emission signal from the valence, or bonding, MOs of the target molecule. By observing the difference in x-ray signals when the incoming photons were polarized horizontally or vertically, the team hoped to find the spatial symmetry of the emitting MO—a parameter that can identify solvent-induced changes to acetic acid’s electronic structure. 

However, detecting symmetry changes in liquids is difficult because the differences between polarized signals are quite small. According to lead author Yuka Horikawa, the team overcame this problem by using a solvent called acetonitrile (CH3CN) that does not interfere with the oxygen x-ray emissions of acetic acid. When the incident x-ray energy was tuned to the oxygen signal, a nitrogen emission from the acetonitrile solvent appeared that was proportional to the incident light intensity, no matter the polarization direction. This nitrogen signal was used to normalize the polarized acetic acid spectra, allowing the solvated symmetry changes to be revealed.

In contrast to expectations, the acetic acid emissions showed pronounced polarization dependence, indicating that the MOs retained the same symmetry as a molecule without solvent. While this result shows that acetonitrile had little effect on most of the compound, one particular MO—corresponding to a lone pair of electrons on the acetic acid oxygen atom—showed a pronounced change. The researchers propose that this change in MO symmetry arises from solvent effects. The new-found ability to precisely pinpoint activation sites has the potential to unlock the secrets of many solvent-based reactions, say the researchers. 

References

  1. Horikawa, Y., Tokushima, T., Hiraya, A. & Shin, S. Pronounced polarization anistropy in resonant X-ray emission from acetic acid molecules in solution. Physical Chemistry Chemical Physics 12, 9165 –9168 (2010). (Link)