Ab initio electronic structure calculations and variational transition state theory are used to calculate reaction
energetics and rate constants for the gas-phase reactions of OH- with CH(4
-
n
)Cl
n
for n = 1−4. Two reaction
pathways are considered, second-order (bimolecular) nucleophilic substitution (SN2), and proton transfer.
Benchmark electronic structure calculations using CCSD(T) and basis sets as large as aug-cc-pVQZ are
performed to obtain highly accurate estimates of the enthalpies of reaction. These results are extrapolated to
the complete basis set limit for comparison with experiment and to establish the level of theory needed to
provide energies that are accurate to better than a few kJ/mol. Energies of critical geometries (reactant
complexes, saddle points, and product complexes) are computed for all systems. For the SN2 reaction, the
potential energy and its first and second derivatives along minimum energy paths are computed and used
directly in variational transition state theory (VTST) calculations of the rate constants. These calculations
indicate that for n = 1−3 the region of the potential in the asymptotic reactant channel controls the reaction
rate constants and that the loose-transition-state methods implemented in variflex provide the best estimates
of the reaction rate constants. The reaction with n = 4 has a dynamical bottleneck that lies near the saddle
point and is best treated using the VTST methods implemented in polyrate.
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