Quantum Dynamics of Gas‐Phase S_N2 Reactions

Abstract

Understanding the state-resolved dynamics of elementary chemical reactions involving polyatomic molecules, such as the well-known reaction mechanism of nucleophilic bimolecular substitution (SN2), is one of the principal goals in chemistry. In this Review, the progress in the quantum mechanical treatment of SN2 reactions in the gas phase is reviewed. The potential energy profile of this class of reactions is characterized by two relatively deep wells, which correspond to pre- and post-reaction chargedipole complexes. As a consequence, the complex-forming reaction is dominated by Feshbach resonances. Calculations in the energetic continuum constitute a major challenge because the high density of resonance states imposes considerable requirements on the convergence and the energetic resolution of the scattering data. However, the effort is rewarding because new insights into the details of multimode quantum dynamics of elementary chemical reactions can be obtained.

Ab initio quantum mechanical calculation of the reaction probability for the reaction

“…A possible extension of the present model would therefore be to further increase the dimensionality of the potential energy surface [32] and in the quantum dynamics. [4] There are seven additional degrees of freedom, and the question is whether any of them would have a critical effect on the calculated reaction rates. Out of the nine vibrational frequencies of the transition complex, see the Supporting information, the antisymmetric and symmetric Cl-P-Cl stretches are already included in the two-dimensional model.…”

“…[1,2] Specially the nucleophilic substitution at carbon centers (S N 2@C) have been extensively studied, both theoretically and experimentally. [3,4] The gas phase substitution reaction between a halide anion and a halomethane,…”

“…A full quantum dynamical simulation of this reaction, with N = 5 atoms and N d = 9 degrees of freedom, is, in practice, very difficult. [4,14,15] It would require the handling of a 10 9 dimensional wavefunction, assuming 10 basis functions or grid points per dimension would be sufficient. Fulldimensional calculations of systems of this size have mainly been performed for reactions with a single heavy atom.…”

Dynamical barrier and isotope effects in the simplest substitution reaction via Walden inversion mechanism

“…The bimolecular nucleophilic substitution (S N 2) reaction is the most common reaction of that type, in which a nucleophile (often negatively charged) approaches a saturated carbon from one side, displaces a leaving group on the opposite side of the carbon atom, resulting in inversion of the carbon centre. Strong solvent effects of these reactions in solution have prompted investigations of the gas-phase S N 2 reaction to probe the intrinsic reaction mechanisms without solvent123456789101112131415161718192021222324252627. Numerous experimental and theoretical studies have revealed that an inverse secondary kinetic isotope effect (KIE), that is, k H / k D <1 (where k denotes the thermal rate constant), is characteristic for a thermal S N 2 reaction when the isotopically substituted atom is not directly involved in a reaction12345678910.…”

“…Furthermore, they found that the reaction thresholds for S N 2 reactions, which are larger than their corresponding barrier heights, result from direct reactions1517. Many reduced dimensionality quantum scattering studies were also carried out to investigate dynamics in S N 2 reactions19202122. Recently, the molecular beam experiments in the Wester's group, in combination with theory, have revealed unprecedented dynamical details on some exothermic S N 2 reactions2324252627.…”