Ultra-fast spectroscopy tracks changes during polyatomic reactions

Japanese and Israeli scientists have developed a technique that can track whole-molecule changes that occur during extremely rapid cis-trans isomerisation reactions. They say the approach reveals, for the first time, exactly how each part of a reactant molecule moves to form  the product.

The researchers used ultra-fast Raman spectroscopy to observe how a well-studied molecule called cis-stilbene is converted into its trans isomer. This light-induced reaction takes less than a picosecond - one millionth of a millionth of a second. Previously, the conversion was thought to result from a simple change in rotation of the molecule’s central double bond.


Source: © Science

Raman reveals that stilbene isomerisation isn’t as simple as once thought

Tahei Tahara at the Institute of Physical and Chemical Research (RIKEN) in Saitama, Japan, who led the team, says the study differs from previous work because it monitors the entire transition from cis  to trans isomer. ’In other work, they just observe the disappearance of the reactant and the appearance of the product,’ he says.

According to Tahara, their research records a more accurate - and complicated - version of events. By firing ultra-short (sub-pico- or femtosecond range) laser pulses at the molecule, the researchers were able to first kick-start the reaction and then take a series of "snapshots" of it. Ordered in sequence, these snapshots show how vibrations in the molecule change over time, revealing that the phenyl rings and hydrogen atoms in stilbene twist to help force the molecule into its new conformation.

Although ultra-short laser pulses have been used before to study light-induced reactions, the novelty of Tahara’s approach is the way he precisely times the pulses to extract information about the molecule’s larger scale, lower frequency vibrational "wobbles" during the reaction.

These low frequency vibrations are important in the reaction of polyatomic molecules, says Richard Mathies, a laser spectroscopy expert at the University of California, Berkeley, US. Mathies thinks Tahara’s work heralds a renaissance in the application of Raman spectroscopy for studying reactions in more complex molecules. ’This is a really nice example of what is going to be a huge advance in studies of chemical reaction dynamics,’ he says.

Tahara notes that the method could be used to validate theoretical calculations for the reactions of large molecules. ’You can show how a molecule changes during a reaction using computer graphics, but it’s not science,’ he says. ’This is a kind of first checking point, which we expect more and more people will use.’

Mathies adds that the technique is not only applicable to light-induced reactions. ’Chemists are figuring out how to use optically mediated initiators to trigger a wide variety of reactions, so the application is potentially very broad.’

Hayley Birch

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