Abstract
The fragment of the 1-methylpyrene cation, C17H+11,<math><mrow><msub><mtext>C</mtext><mrow><mn>17</mn></mrow></msub><msubsup><mtext>H</mtext><mrow><mn>11</mn></mrow><mo>+</mo></msubsup><mo>,</mo></mrow></math> is expected to exist in two isomeric forms, 1-pyrenemethylium PyrCH+2<math><mrow><msubsup><mrow><mtext>PyrCH</mtext></mrow><mn>2</mn><mo>+</mo></msubsup></mrow></math> and the tropylium containing species PyrC+7.<math><mrow><msubsup><mrow><mtext>PyrC</mtext></mrow><mn>7</mn><mo>+</mo></msubsup><mo>.</mo></mrow></math> We measured the infrared (IR) action spectrum of cold C17H+11<math><mrow><msub><mtext>C</mtext><mrow><mn>17</mn></mrow></msub><msubsup><mtext>H</mtext><mrow><mn>11</mn></mrow><mo>+</mo></msubsup></mrow></math> 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 PyrCH+2<math><mrow><msubsup><mrow><mtext>PyrCH</mtext></mrow><mn>2</mn><mo>+</mo></msubsup></mrow></math> 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.