Coupled Unimolecular Dissociation Kinetics of Bromotoluene
Radical Cations
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Abstract
The unimolecular dissociations of <i>o</i>-, <i>m</i>-, and <i>p</i>-bromotoluene
radical cations
to C<sub>7</sub>H<sub>7</sub><sup>+</sup> (benzylium and tropylium)
are examined by considering the coupling of the three isomers in the
dissociation pathways. The potential energy surface obtained from
ab initio calculations suggests the interconversion of isomers through
methylene and hydrogen migrations on the ring. The rate equations
for each isomer are combined together to form a rate matrix for coupled
reactions. The rate matrix contains the microcanonical rate constants
for all elementary steps, which are calculated using Rice–Ramsperger–Kassel–Marcus
theory based on the molecular parameters obtained from density functional
theory. The unimolecular dissociation rates for coupled reactions
are determined by numerically solving the matrix equation. As a result
of reaction coupling, the product branching ratio becomes time-dependent
and the reaction rates of three isomers become parallel to one another
as the energy increases, although their initial rates differently
vary with energy. The calculated rate–energy curves fall below
the time-resolved photodissociation data in the energy range 2.2–2.7
eV but are in line with the photoelectron photoion coincidence data
in the energy range 2.7–3.5 eV. The discrepancy between experiment
and theory in the low-energy region is ascribed to the uncertainties
of the potential energy surface as well as the contribution of the
radiative relaxation rate that has not been taken into account in
the theoretical calculations. The rate–energy curves are then
used to calculate the thermal reaction rate constants, and the Arrhenius
parameters are determined in the temperature range 700–1300
K. Comparison of the activation energy and entropy obtained from the
Arrhenius plot with the calculated enthalpy and entropy changes between
the reactant and the highest-lying transition state suggests that
a series of [1,2] H-atom migrations occurring near the entrance comprise
the rate-determining steps and the subsequent [1,2] H-atom migrations
play an important role in increasing the activation energy and decreasing
the entropy by reducing the net flux to the exit