## Abstract

Chemical dynamics simulations, based on both an analytic potential
energy surface (PES) and direct dynamics, were used to investigate the
intrinsic non-RRKM dynamics of the Cl^{−}–CH_{3}Br ion-dipole
complex, an important intermediate in the Cl^{−} + CH_{3}Br
S_{N}2 nucleophilic substitution reaction. This intermediate
may dissociate to Cl^{−} + CH_{3}Br or isomerize to the
ClCH_{3}–Br^{−} ion-dipole complex. The decomposition of
microcanonical ensembles of the Cl^{−}–CH_{3}Br intermediate were
simulated, and the ensuing populations *vs.* time of the
excited intermediate and Cl^{−} + CH_{3}Br and
ClCH_{3}–Br^{−} products were fit with multi-exponential
functions. The intrinsic non-RRKM dynamics is more pronounced for the
simulations with the analytic PES than by direct dynamics, with the
populations for the former and latter primarily represented by tri-
and bi-exponential functions, respectively. For the analytic PES and
direct dynamics simulations, the intrinsic non-RRKM dynamics is more
important for the isomerization pathway to form ClCH_{3}–Br^{−}
than for dissociation to Cl^{−} + CH_{3}Br. Since the
decomposition probability of Cl^{−}–CH_{3}Br is non-exponential,
the Cl^{−}–CH_{3}Br unimolecular rate constant depends on
pressure, with both high and low pressure limits. The high pressure
limit is the RRKM rate constant and for the simulations with the
analytic PES the rate constant decreased by a factor of 3.0, 5.6, and
4.3 in going from the high to low pressure limit for total energies of
40, 60, and 80 kcal/mol. For the direct dynamics
simulations these respective factors are 2.4, 1.4, and
1.2. A separable phase space model with intermolecular and
intramolecular complexes describes some of the simulation results, but
overall models advanced for intrinsic non-RRKM dynamics give
incomplete representations of the intermediate and product populations
*vs.* time determined from the simulations.

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