When molecules take the plunge into a liquid solvent, they undergo constant twists
\nand turns as they spread out and interact within their new environment. Such
\nhaphazard movements in solvents make it difficult for scientists to measure
\n specific reactivity changes. Especially for liquids and solutions, it is not known
\nhow intermolecular interactions affect the electronic structure of the reactants,
\ndespite this being one of the key physical processes of chemistry.
\n.
\nNow, scientists from RIKEN Institute and Hiroshima University have used high-energy
\n synchrotron light to capture the signals of molecular orbitals (MOs)\u2014quantized
\nspatial distributions of electrons that determine chemical reactivity\u2014from acetic
\nacid molecules in solution1 (Fig. 1). This approach enables the measurement of
\nsolvation effects with atom-by-atom precision, which is crucial information for
\nunderstanding essential reactions such as enzyme-based catalysis.
\n.
\nThe researchers achieved their result by smashing accelerated photons into an
\nacetic acid solution, setting off an x-ray emission signal from the valence, or
\nbonding, MOs of the target molecule. By observing the difference in x-ray signals
\n when the incoming photons were polarized horizontally or vertically, the team
\n hoped to find the spatial symmetry of the emitting MO\u2014a parameter that can
\n identify solvent-induced changes to acetic acid\u2019s electronic structure.
\n.
\nHowever, detecting symmetry changes in liquids is difficult because the differences
\nbetween polarized signals are quite small. The team overcame this problem by using
\n a solvent called acetonitrile (CH3CN) that does not interfere with the oxygen x-ray
\nemissions of acetic acid. When the incident x-ray energy was tuned to the oxygen
\n signal, a nitrogen emission from the acetonitrile solvent appeared that was proportional
\n to the incident light intensity, no matter the polarization direction. This nitrogen signal
\n was used to normalize the polarized acetic acid spectra, allowing the solvated symmetry
\nchanges to be revealed.
\n.
\nIn contrast to expectations, the acetic acid emissions showed pronounced polarization dependence,
\nindicating that the MOs retained the same symmetry as a molecule without solvent. While this
\nresult shows that acetonitrile had little effect on most of the compound, one particular MO\u2014corresponding
\nto a lone pair of electrons on the acetic acid oxygen atom\u2014showed a pronounced change.
\nThe researchers propose that this change in MO symmetry arises from solvent effects. The new-found
\nability to precisely pinpoint activation sites has the potential to unlock the secrets of many solvent-based
\nreactions, say the researchers. <\/p>\n","protected":false},"excerpt":{"rendered":"
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 … <\/p>\n