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

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.
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Now, scientists from RIKEN Institute 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.
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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.
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However, detecting symmetry changes in liquids is difficult because the differences
between polarized signals are quite small. 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.
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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.