The benzylium and tropylium cations identified as two stable C7H7+ isomers

The picture, which is a cover of the journal ChemPhysChem, schematizes the production of the C7H7 ion by dissociative ionization of toluene and the identification, by infrared spectroscopy, of the two isomers produced: the cations benzylium (right) and tropylium (left).

Experiments driven by an international team involving the Institute for Research in Astrophysics and Planetary Sciences (IRAP, Université de Toulouse/CNRS) and the Laboratory of Quantum Chemistry and Physics (LCPQ, Université de Toulouse/CNRS), enabled to identify the two stable isomers of C7H7+ ion.

The ion C7H7is a well-known species in mass spectrometry, formed by ionization of hydrocarbons such as toluene. The two most stable structures proposed for this ion are benzylium and tropylium ions. The first (a benzene with a methylene group) could be identified by its chemical reactivity, but is not the case of the second, whose structure consisting of an aromatic cycle of 7 carbons is predicted by quantum chemistry calculations.

Two structures have been identified for C7H7+ and the vibrational spectra obtained are in agreement with those of benzylium and tropylium ions calculated with the density functional theory. In addition, measures in depletion helped to show that no other isomer was present and this for different precursors used in the production of C7H7+.

The study took advantage of the FELion line, installed on the free electron laser FELIX, in the Netherlands. FELion includes a cryogenic ion trap that allows attaching an atom of rare gas on the ions studied. This technique of tagging allows to implement a spectroscopy of action by dissociating the complexion/atom of rare gas with a single infrared photon unlike the technique usually used of multiple absorption of photons to attain the threshold of dissociation of the ion. This technique has the advantage to probe ions without heating them and so without disrupting their structure through processes of isomerization.

This work of identification of C7H7+ isomers opens up prospects for the study of the growth paths of the hydrocarbon ions in complex environments both on Earth (chemistry of flames, and plasmas) or space (interstellar chemistry and planetary atmospheres as Titan’s).

This interdisciplinary work (INSU/INP/INC) has been initiated as part of the ERC Synergy NANOCOSMOS project in collaboration with the CSIC (Madrid) and involves a collaboration between the universities of Toulouse and Cologne as part of the European Training Network (ETN) EUROPAH.

Scientific paper: 

P. Jusko, A. Simon, S.Banhatti, S. Brünken, C. Joblin (2018) DirectEvidence of the Benzylium and Tropylium Cations as the two long-lived Isomersof C7H7+, ChemPhysChem, doi:10.1002/cphc.201800744 – Archives ouvertes: hal-01880438, arXiv:1809.09375

Link to the INSU news page: Les cations de benzylium et tropylium identifiés comme les deux isomères stables de C7H7+