Time-dependent mass spectra and breakdown graphs .20. Bromoanthracene. Heat of formation of the anthracenyl ion

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 C14H9+. 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: Delta H(f0)degrees(C14H9+ anthracenyl) = 288.8 +/- 3 kcal mol(-1), Delta H(f0)degrees(C14H8 .(+)) less than or equal to 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.