Identification of the fragment of the 1-methylpyrene cation by mid-IR spectroscopy

The fragment of the 1-methylpyrene cation, ${\text{C}}_{17}{\text{H}}_{11}^{+},$ is expected to exist in two isomeric forms, 1-pyrenemethylium ${\text{PyrCH}}_2^{+}$ and the tropylium containing species ${\text{PyrC}}_7^{+}.$ We measured the infrared (IR) action spectrum of cold ${\text{C}}_{17}{\text{H}}_{11}^{+}$ tagged with Ne using a cryogenic ion trap instrument coupled to the FELIX laser. Comparison of the experimental data with density functional theory calculations allows us to identify the ${\text{PyrCH}}_2^{+}$ isomer in our experiments. The IR Multi-Photon Dissociation spectrum was also recorded following the C₂H₂ loss channel. Its analysis suggests combined effects of anharmonicity and isomerisation while heating the trapped ions, as shown by molecular dynamics simulations.