Time-dependent mass spectra and breakdown graphs. 18. pyrene

Unimolecular fragmentations of the pyrene radical cation were studied by time-resolved photo-ionization in the VUV, RRKM/QET calculations and MS/MS with electron ionization. The major reactions observed are parallel H• and H2 losses, as well as consecutive H• losses. Activation parameters were deduced for the reactions. Appearance energies were determined for the microsecond and millisecond time ranges. The reactions demonstrate large conventional and intrinsic shifts. The H• loss reactions are characterized by loose transition states and have no reverse activation energies. This has enabled the determination of new thermochemical data. The C–H bond energy in the pyrene radical cation was deduced to be 4.6 eV and the heat of formation of the pyrenyl cation, ΔHf00(Cl6H9+) = 1190 kJ mol−1. The results are discussed in light of the possible role of pyrene-like ions in interstellar space.