Time-resolved photodissociation of p-bromotoluene ion as a probe of ion internal energy

Abstract

The unimolecular dissociation of the p-bromotoluene radical cation was studied by using Fourier transform ion cyclotron resonance spectrometry. The parent ions were prepared by charge-transfer reactions of toluene- d 8 ions with p-bromotoluene. The toluene- d 8 radical cations were produced by two-photon ionization at 266 nm. The photodissociation was accomplished by 532 nm. The p-bromotoluene ion dissociates to C 7H 7 + with a loss of Br via one- and two-photon processes. The structures of the C 7H 7 + products were identified by ion-molecule reactions. The dissociation rates were measured by time-resolved photo-dissociation (TRPD) spectroscopy. The C 7H 7 + products from one-photon dissociation (1PD) were reactive toward toluene- d 8 and yielded CD 3C 6D 4CH 2 +, indicating the benzyl ion structure. No unreactive tropylium ions were detected from 1PD. The rate constant of 1PD obtained at the internal energy of 2.43 eV was combined with the photoelectron-photoion coincidence data reported in the internal energy range 2.75–3.33 eV to estimate the activation parameters for the lowest barrier process that led to the benzyl ion exclusively. The two-photon dissociation (2PD) resulted in both the benzyl and tropylium ions with the branching ratio, [benzyl +]/[tropylium +] = 92/8. It appears that the tropylium ion results from the isomerization of the p-tolyl ion produced by 2PD via a direct C-Br cleavage. The TRPD spectroscopy was used as a thermometric probe of the average internal energy of the parent ions produced by electron impact. The rates of 1PD were compared as a function of cooling time and pressure to examine the energy cooling processes. The effect of radiative heating of an ICR cell by hot filament on the dissociation rate was also examined.