The sequence of coronene hydrogenation revealed by gas-phase IR spectroscopy

Gas-phase coronene cations (C₂₄H₁₂⁺) can be sequentially hydrogenated with up to 24 additional H atoms, inducing a gradual transition from a planar, aromatic molecule towards a corrugated, aliphatic species. The mass spectra of hydrogenated coronene cations [C₂₄H₁₂+nH ]⁺ show that molecules with odd numbers of additional hydrogen atoms (nH) are dominant with particularly high relative intensity for ”magic numbers” nH = 5, 11, and 17, for which hydrogen atoms have the highest binding energies. Reaction barriers and binding energies strongly affect the hydrogenation sequence and its site specificity. In this contribution, we monitor this sequence experimentally by the evolution of infrared multiple-photon dissociation (IRMPD) spectra of gaseous [C₂₄H₁₂+nH]⁺ with nH = 3 − 11, obtained using an infrared free electron laser coupled to a Fourier transform ion cyclotron mass spectrometer.
For weakly hydrogenated systems (nH = 3, 5) multiple-photon absorption mainly leads to loss of Hatoms (and/or H₂). With increasing nH, C₂H₂ loss becomes more relevant. For nH = 9, 11, the carbon skeleton is substantially weakened and fragmentation is distributed over a large number of channels. A comparison of our IRMPD spectra with density functional theory calculations clearly shows that only one or two hydrogenation isomers contribute for each nH. This confirms the concept of hydrogenation
occurring along very specific sequences. Moreover, the atomic sites participating in the first 11 steps of this hydrogenation sequence are clearly identified.