Effects of reaction path curvature on reaction dynamics and rates: Reaction path Hamiltonian calculations for gas-phase SN2 reaction Cl?+CH3Cl

Abstract

To obtain essential information on the reaction dynamics for the prototype gas-phase S 2 reaction Cl y q CH Cl ª ClCH q Cl y , the characteristic N 3 3 features of the potential energy surface in the local region around the reaction path were examined by the reaction path Hamiltonian constructed with high-level ab initio molecular-orbital calculations. After the structures of relevant stationary states and the intrinsic reaction coordinate were determined, the transverse vibrational modes, the corresponding frequencies, and the coupling elements between the pairs of normal modes induced by the reaction coordinate motion were calculated at each point along the intrinsic reaction coordinate. It was found that a quite large reaction path curvature appears in the intrinsic barrier slope near the bottom of each of the pre-and postreaction stable-state complexes. This large curvature was clarified to cause the internal vibrational excitation of the products and the requirement of the vibrational excitation of the reactants for reaction occurrence. The complex recrossings across the transition-state w theory dividing surface, previously characterized by Hase et al. J. Chem. Phys. 96, 8275 Ž .x y 1992 in which trajectories trapped in the Cl иииCH Cl complex return to the central 3 barrier region, were demonstrated to be attributed to this large curvature. Furthermore, not only the variational effects but also the reaction path curvature effects on the intermediate recrossings that were also characterized by Hase et al., in which trajectories linger near the central barrier, were found to be negligible.

Model Identity SN2 Reactions CH3X + X- (X = F, Cl, CN, OH, SH, NH2, PH2): Marcus Theory Analyzed

“…We begin the present analysis with the identity reaction CH 3 Cl + Cl - , with the ion−molecule complex and transition state shown in Figure . Along with the fluoride identity reaction, this is among the most studied of all S N 2 reactions. ,,,,,,− The best previous studies have utilized CCSD(T), QCISD(T), and the W2 42 extrapolation schemes. The ion−molecule complex is a classic backside complex, in C 3 v symmetry, with collinear heavy atoms and a large Cl−C distance of 3.151 Å.…”

Chemical Dynamics Simulations of X– + CH3Y → XCH3 + Y– Gas-Phase SN2 Nucleophilic Substitution Reactions. Nonstatistical Dynamics and Nontraditional Reaction Mechanisms

“…(Harmonic ab initio frequencies were multiplied by scale factors to obtain the anharmonic frequencies. For Cl – + CH 3 Br CCSD­(T)/PP/aug-cc-pVQZ harmonic frequencies were used and MP2 theory, with different basis sets, was used to determine the harmonic frequencies for the remaining reactions, i.e., aug-cc-pVTZ, 6-311++G­(2d,p), QZ­(+)­(2d,2p), and 6-311++G­(2df,2pd) for Cl – + CH 3 I, Cl – + CH 3 Cl, F – + CH 3 F, and F – + CH 3 Cl, respectively. The scale factors are taken from ref ).…”

Ab initio and DFT benchmark study for nucleophilic substitution at carbon (SN2@C) and silicon (SN2@Si)

“…Bimolecular nucleophilic substitution (S N 2) constitutes a class of elementary chemical reactions that play an important role in organic chemistry 1–3. Various theoretical4–21 and experimental22–32 studies have been conducted to obtain a detailed description of the potential energy surface (PES) of S N 2 reactions. The symmetric, thermoneutral S N 2 reaction between the chloride anion and chloromethane in the gas phase is generally used as the archetypal model for nucleophilic substitution [see eq.…”