Laser measurements of the unimolecular kinetics of energy-selected molecular ions: Isotope effects in benzene

Metastable ion decay rates for isotope decay channels differing only by one mass unit have been measured in a reflectron mass spectrometer with high mass resolution. Several decay channels with H, H2, C2H2, and C3H3 (and their isotopic analogs) elimination from C6H+6, C6D+6, and C6D5H+ are investigated. We present examples of intermolecular and for the first time of intramolecular kinetic isotope effects for internal energy selected reactants. The reaction coordinate for the decay channels can be characterized as C–H or alternatively as C–C stretching vibrations and the total looseness of the activated complex with respect to the remaining 3N−7 vibrations can be attributed to the subsets of the C–H and C–C vibrations. Furthermore, from the good agreement of RRKM calculations and all inter- and intramolecular isotopic results for a given decay channel it is concluded that C–H vibration participate in the random energy flow before dissociation.