Benchmark measurement of iodobenzene ion fragmentation rates

Abstract

The unimolecular fragmentation rate of iodobenzene ion has been studied by variable residence time photoelectronphotoion coincidence techniques. The techniques employed variable wavelength with threshold photoelectron detection and fixed (58.4 nm) wavelength with variable energy photoelectron detection, respectively. Residence times of 1.0 = 0.25 or 5.9 ± 0.3 and 21 ± 1 or 57 ± 1 μs were employed. The four sets of measurements were independently analyzed using exact counting of harmonic oscillator states, taking into account the appropriate (and different) apparatus functions and the thermal energy distributions of the parent ions. The resulting rate-energy dependences and fragmentation threshold values were in excellent agreement with one another. The best-fit rate-energy dependence is proposed as a benchmark for calibration of future rate-energy measurements. The resulting Δ H f D 0 (C 6H 5 +) = 1133 ± 5 kJ mol is in excellent agreement with earlier results based on a somewhat simpler method of analysis of chlorobenzene and bromobenzene fragmentation rates. Some remaining uncertainties regarding the transition-state model are discussed.