Two of a kind – unraveling isomeric structures using FELIX

What is it about?

Researchers from the University of Toulouse/CNRS, the University of Cologne and FELIX Laboratory successfully disentangled the isomeric structure of C7H7+. By using gas-phase infrared spectroscopy they not only found specific production pathways to preferentially form one or the other isomer, but could also exclude the existence of any other long-lived isomer. This work opens perspectives for a better understanding of the formation paths leading to either tropylium or benzylium ions, and can also be applied to other isomeric systems.

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Why is it important?

Disentangling the isomeric structure of C7H7+, a common product when you ionize alkylbenzenes and other hydrocarbons, is a longstanding experimental issue. The presence of at least two isomers, tropylium (a very stable 7-membered aromatic ring) and benzylium (a six membered ring with an attached CH2 group), was presumed from previous studies. In these earlier mass spectrometry studies a difference in reactivity was used to identify the fraction of benzylium formed. However, the presence of the tropylium ion could only be indirectly inferred. Using the FELion cryogenic ion trap beamline at the free electron laser FELIX, the research team used infrared spectroscopy to assign the contributions of benzylium and tropylium isomers. They took advantage of the possibility to tag the species of interest in the cold trap in order to monitor their spectra at low temperature. This allows them to identify the two isomers and selectively isolate one or the other. With this technique, the abundance of each isomer could be tracked.