Ultrafast Raman spectroscopy can now be used to study the global motions of reacting polyatomic molecules and reveals how stilbene twists as it photoisomerizes
Spectroscopists have been studying reactions for many years to understand rates and mechanisms, but less so to understand the motions of the molecule as they change from reactants to products. The photoexcited isomerization of cis-stilbene to its trans-isomer — a reaction well studied spectroscopically — seemingly proceeds by a simple rotation around the central C=C bond. Now a team of scientists from Japan and Israel, led by Tahei Tahara at the Institute of Chemical and Physical Research, RIKEN, have developed1 a method to probe the motions of the whole molecule as it isomerizes, rather than just the C=C bond.
The researchers used an ultrafast pump–probe method that introduces an extra third pulse, between the pump and probe pulses, which enables the detection and tracking of low-frequency vibrations that are usually inaccessible. They monitored a vibrational 'spectator' mode that shifts in frequency as the isomerization proceeds, and used density functional theory calculations to attribute the spectator mode to specific structural changes during the reaction.
The experiments and calculations show how the motions of the whole molecule evolve, starting with a rapid stretching of the central C=C bond and out-of-plane motion of the ethylenic hydrogen atoms. This is followed by a slower movement of these hydrogen atoms that helps to twist the C=C bond and results in the trans-stilbene configuration. In the future the method could be applied to larger molecules, such as proteins, following their molecular choreography as they react.
Takeuchi, S. et al. Spectroscopic tracking of structural evolution in ultrafast stilbene photoisomerization. Science 322, 1073–1077 (2008).
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Armstrong, G. A new twist. Nature Chem (2008). https://doi.org/10.1038/nchem.94