Time-dependent mass spectra and breakdown graphs: 20: bromoanthracene: heat of formation of anthracenyl ion

Abstract

Unimolecular fragmentations of the 9-bromoanthracene radical cation were studied by time-resolved photoionization mass spectrometry (TPIMS) in the vacuum-UV and by RRKM/QET calculation. The vibrational frequencies required in the RRKM calculation were obtained by density functional calculations. The major reactions observed are Br- loss and consecutive H- loss from C14H+9. Appearance energies (AEs) were determined for the microsecond and millisecond time ranges. Activation parameters were deduced for the reactions. The Br- loss reaction is characterized by a loose transition state. The following heats of formation were deduced: Δ10°(C14H+9, anthracenyl) = 288.8 ± 3 kcal mol−1, ΔH10°(C14H8·+) ≤ 297.2 ± 3 kcal mol−1. The C-Br bond energy in the bromoanthracene radical cation is 3.30 ± 0.08 eV. The effect of the ring system size on the bond energies of polycyclic aromatic hydrocarbons (PAHs) is discussed.