Quantum dynamics of the Walden inversion reaction Cl−+CH3Cl→ClCH3+Cl−

Abstract

Quantum scattering calculations on the SN2 reaction Cl−+CH3Cl→ClCH3+Cl− are reported. The rotating bond approximation (RBA) has been adapted so that three degrees of freedom including the C–Cl stretching vibration and the CH3 umbrella mode are treated explicitly. The calculations have been done with minor modifications of a potential due to Vande Linde and Hase. It is found that initial excitation of the C–Cl vibration has a large effect on the reaction probabilities, while excitation of the CH3 umbrella vibration is less significant. The reaction is dominated by scattering resonances with lifetimes ranging from 0.1 to 10 ps. It is found that the length of the C–Cl bond at the transition state of the reaction has a particularly pronounced effect on the reaction probabilities. The magnitude of the quantum reaction probabilities compares quite well with those calculated using the quasiclassical trajectory method.

Theoretical studies on bimolecular reaction dynamics

“…For example reduced dimensional calculations on the classic S N 2 reaction Cl Ϫ ϩ CH 3 Cl 3 ClCH 3 ϩ Cl Ϫ suggest that reaction occurs via many scattering resonances correlating to excited ion-dipole complexes (39,40). However, in probing the dynamics of these and more complicated reactions the use of quantum dynamics methods is limited because of the number of states that need to be included in the computations and it is often necessary to resort to methods based on classical dynamics.…”

Quantum Dynamics of Gas‐Phase SN2 Reactions

“…The PESs were used in extensive classical dynamics calculations by Hase and co-workers [5, 43±56] and were also employed in many of the quantum-dynamical calculations reviewed in this work. Clary and co-workers [38,60] scaled the geometry of the saddle point in Hase's PESs without changing the barrier heights, and obtained, in their quantumdynamics calculations, a somewhat better agreement with experiment (see Section 3). Tucker and Truhlar [61] also provided a six-body PES (based on second order M˘ller±Plesset, MP2/6±31G(d,p), calculations) for reaction (2), which was later slightly modified by Billing.…”

“…[72] The other two remaining two-fold degenerate modes, the high-frequency asymmetric CÀH stretch and the CH 3 bend, should not be as important for the overall reaction dynamics. [60] However, the two degenerate ClÀC bending modes, both for the weak intermolecular bond and the intramolecular bond, are considered to be more significant. Since the weak modes are important with respect to energy transfer (T!R, T!V), they should be included in both channels, which would result in at least six degrees of freedom, the explicit and accurate quantum treatment of which is not feasible at present.…”

“…[95] It is anticipated that the excitation of this mode also makes approaching the saddle-point molecular geometry easier. Clary and Palma [60] used an approximate Hamiltonian to study the reaction [Equation (7)]:…”

Computational methods of quantum reaction dynamics

“…[36,37,79] Still, four-atom reactions are mostly treated with reduced dimensionality approaches. [45,46,74,[80][81][82][83][84] Time-Dependent Quantum Dynamics…”