IR photofragmentation of the Phenyl Cation: Spectroscopy and Fragmentation Pathways

We present the gas-phase infrared spectra of the phenyl cation, phenylium, in its perprotio C$_6$H$_5^+$ and perdeutero C$_6$D$_5^+$ forms, in the 260-1925 cm$^{-1}$ (5.2-38 $\mu$m) spectral range, and investigate the observed photofragmentation. The spectral and fragmentation data were obtained using Infrared Multiple Photon Dissociation (IRMPD) spectroscopy within a Fourier Transform Ion Cyclotron Resonance Mass Spectrometer (FTICR MS) located inside the cavity of the free electron laser FELICE (Free Electron Laser for Intra-Cavity Experiments). The $^1$A$_1$ singlet nature of the phenylium ion is ascertained by comparison of the observed IR spectrum with DFT calculations, using both harmonic and anharmonic frequency calculations. To investigate the observed loss of predominantly [2C,nH] (n=2-4) fragments, we explored the potential energy surface (PES) to unravel possible isomerization and fragmentation reaction pathways. The lowest energy pathways toward fragmentation include direct H elimination, and a combination of facile ring-opening mechanisms ($\leq2.4$ eV), followed by elimination of H or CCH$_2$. Energetically, all H-loss channels found are more easily accessible than CCH$_2$-loss. Calculations of the vibrational density of states for the various intermediates show that at high internal energies, ring opening is the thermodynamically the most advantageous, eliminating direct H-loss as a competing process. The observed loss of primarily [2C,2H] can be explained through entropy calculations that show favored loss of [2C,2H] at higher internal energies.