Thermal dissociation of acetophenone molecular ions activated by infrared radiation

The thermal dissociation of the molecular ions of acetophenone (C6H5COCH3.+ --> C6H5CO+ + (CH3)-C-.) and acetophenone-d(3) (C6H5COCD3.+ --> C6H5CO+ + (CD3)-C-.) induced by broad band infrared radiation has been studied in the cell of an FT-ICR spectrometer. Rate constants in the range of 0.5-10 s(-1) have been obtained for the system of ions exposed to a radiation source equivalent to blackbody temperatures between 1100 and 1600 K. The unimolecular dissociation is almost pressure independent in the 4 x 10(-8) to 5 x 10(-7) Torr range indicating that the most important mechanism is of a noncollisional nature. Activation energies obtained. from Arrhenius-type plots yield 46.6 +/- 2.0 kJ mol(-1) for acetophenone and 44.9 +/- 2.2 kJ mol(-1) for acetophenone-d(3). The dissociation process has been modeled by a Monte Carlo simulation and by numerical solution of the master equation of a process which takes into account interaction with the background radiation field through absorption and emission. These calculations reveal that meaningful activation energies can be obtained from these experiments even though the exact radiance viewed by the ions is not known. Solution of the master equation reveals that the experimental activation energies are consistent with a dissociation energy of 80.5 W mol(-1) for the acetophenone molecular ion. This result is used to derive a heat of formation of 745 kJ mol(-1) for the C6H5CO+ ion.