Dj. Mann et Wl. Hase, TRAJECTORY STUDIES OF S(N)2 NUCLEOPHILIC-SUBSTITUTION - 6 - TRANSLATIONAL ACTIVATION OF THE CL-+CH3CL REACTION, The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory, 102(31), 1998, pp. 6208-6214
Classical trajectory simulations are used to study the translational a
ctivation of the Cl- + CH3Cl S(N)2 reaction at energies in the range 2
0-80 kcal/mol. The trajectories are calculated on the PES3 analytic po
tential energy surface. The shape of the reactive cross section versus
relative translational energy E-rcl and the translational threshold o
f 18 kcal/mol are both similar to recent experimental results [J. Phys
. Chem. A 1997, 101, 5969]. The reactive trajectories are direct, with
negligible trapping in the ion-dipole complexes. The product energy i
s primarily partitioned to relative translation with small and similar
amounts of energy partitioned to vibration and rotation. The velocity
scattering angle distribution suggests backward scattering and a rebo
und mechanism for translational activation at low E-rel, with increasi
ng importance of forward scattering and a stripping mechanism as E-rel
is increased. An analysis of angular momenta terms and their correlat
ions shows that the total angular momentum is well-approximated by the
initial orbital angular momentum, which is strongly correlated with t
he final orbital angular momentum. The decrease in the reactive cross
section with CD3Cl isotopic substitution is consistent with the experi
ments. The principal difference between the trajectories and experimen
ts is the order of magnitude larger cross sections found from the traj
ectories. No pronounced inadequacies in the PES3 potential energy surf
ace are evident from comparisons with MP2/6-311+G* ab initio calculat
ions.