Specific Rate Constants k(E) of the Dissociation of the Halobenzene Ions: Analysis by Statistical Unimolecular Rate Theories

Abstract

Specific rate constants k(E) of the dissociation of the halobenzene ions C6H5X+ --> C6H5+ + X* (X* = Cl, Br, and I) were measured over a range of 10(3)-10(7) s-1 by threshold photoelectron-photoion coincidence (TPEPICO) spectroscopy. The experimental data were analyzed by various statistical unimolecular rate theories in order to derive the threshold energies E0. Although rigid activated complex RRKM theory fits the data in the experimentally measured energy range, it significantly underestimates E0 for chloro- and bromobenzene. Phase space theory (PST) does not fit the experimentally measured rates. A parametrized version of the variational transition state theory (VTST) as well as a simplified version of the statistical adiabatic channel model (SSACM) incorporating an energy dependent rigidity factor provide excellent fits to the experimental data and predict the correct dissociation energies. Although both approaches have just two adjustable parameters, one of which is E0, SSACM is effective and particularly simple to apply.