TIME-DEPENDENT MASS-SPECTRA AND BREAKDOWN GRAPHS .14. IODOTOLUENE ISOMERS

The experimental methods of (a) TPIMS (time-resolved photoionization mass spectrometry) and ion trapping in radio frequency fields and (b) TRIMS (time-resolved ion momentum spectrometry) with space charge trapping of ions were applied to the reaction C7H7I+ --> C7H7+ + I in p-iodotoluene and m-iodotoluene. Time-resolved photoionization efficiency (PIE) curves and kinetic energy release distributions (KERDs) were determined. The lowest appearance energies observed for both reactants at storage times of approximately 1 ms are 10.6-10.7 eV and lie approximately 0.2-0.3 eV above the calculated thresholds. The excess energy is ascribed partly to an "ordinary" kinetic shift and partly to an "intrinsic" one, the latter reflecting competition to dissociation by radiative relaxation in the IR. The KERDs are bimodal. At storage times of approximately 100-mu-s, reflecting most probable microcanonical rate constants of k(mp) = 10(4) s-1, the dominant process for both isomers leads to a high KER and proceeds via a tight transition state. In m-iodotoluene, the low-KER component becomes dominant at k(mp) = 10(5) s-1. This may be interpreted as a crossing in the rate-energy dependences for tolyl and tropyl ion formation in m-iodotoluene while the curves remain parallel in p-iodotoluene.