Temperature Dependence of the OH^–+ CH₃I Reaction Kinetics. Experimental and Simulation Studies and Atomic-Level Dynamics

Abstract

Direct dynamics simulations and selected ion flow tube (SIFT) experiments were performed to study the kinetics and dynamics of the OH(-) + CH3I reaction versus temperature. This work complements previous direct dynamics simulation and molecular beam ion imaging experiments of this reaction versus reaction collision energy (Xie et al. J. Phys. Chem. A 2013, 117, 7162). The simulations and experiments are in quite good agreement. Both identify the SN2, OH(-) + CH3I → CH3OH + I(-), and proton transfer, OH(-) + CH3I → CH2I(-) + H2O, reactions as having nearly equal importance. In the experiments, the SN2 pathway constitutes 0.64 ± 0.05, 0.56 ± 0.05, 0.51 ± 0.05, and 0.46 ± 0.05 of the total reaction at 210, 300, 400, and 500 K, respectively. For the simulations this fraction is 0.56 ± 0.06, 0.55 ± 0.04, and 0.50 ± 0.05 at 300, 400, and 500 K, respectively. The experimental total reaction rate constant is (2.3 ± 0.6) × 10(-9), (1.7 ± 0.4) × 10(-9), (1.9 ± 0.5) × 10(-9), and (1.8 ± 0.5) × 10(-9) cm(3) s(-1) at 210, 300, 400, and 500 K, respectively, which is approximately 25% smaller than the collision capture value. The simulation values for this rate constant are (1.7 ± 0.2) × 10(-9), (1.8 ± 0.1) × 10(-9), and (1.6 ± 0.1) × 10(-9) cm(3)s(-1) at 300, 400, and 500 K. From the simulations, direct rebound and stripping mechanisms as well as multiple indirect mechanisms are identified as the atomic-level reaction mechanisms for both the SN2 and proton-transfer pathways. For the SN2 reaction the direct and indirect mechanisms have nearly equal probabilities; the direct mechanisms are slightly more probable, and direct rebound is more important than direct stripping. For the proton-transfer pathway the indirect mechanisms are more important than the direct mechanisms, and stripping is significantly more important than rebound for the latter. Calculations were performed with the OH(-) quantum number J equal to 0, 3, and 6 to investigate the effect of OH(-) rotational excitation on the OH(-) + CH3I reaction dynamics. The overall reaction probability and the probabilities for the SN2 and proton-transfer pathways have little dependence on J. Possible effects on the atomistic mechanisms were investigated for the SN2 pathway and the probability of the direct rebound mechanism increased with J. However, the other atomistic mechanisms were not appreciably affected by J.

Effect of higher order solvation and temperature on SN2 and E2 reactivity

“…The tendency to form unsolvated products is attributed to the geometries of the transition state and the post-reaction complex [19]. The traditional image of an S N 2 reaction is that it proceeds through a "backside attack" in which the reactant nucleophile attacks the carbon from the opposite side of the leaving group, although the Hase group has shown that other mechanisms also occur [28]. The E2 mechanism proceeds through an anti-periplanar transition state.…”

Benchmark ab initio and dynamical characterization of the stationary points of reactive atom + alkane and SN2 potential energy surfaces

“…13 As mentioned in the Introduction we developed 43 the first full-dimensional high-level ab initio analytical PES for the F À + CH 3 Cl S N 2 reaction in 2013 and later we reported 44,45 analytical PESs for other S N 2 reactions as well. Unlike the traditional direct dynamics studies, 9,41,42,52,108,[122][123][124][125][126] the analytical PESs made the computations of millions of trajectories possible, allowing the discovery of low-probability reaction channels and determination of statistically accurate differential cross sections. Therefore, the analytical PESs played a key role in revealing a new reaction mechanism, called double inversion, 46 for S N 2 reactions and achieving unprecedented agreement between theory and detailed crossed-beam experiments.…”

“…TOP revealed that F À spends significant time in the front-side complex region of the F À + CH 3 I reaction, whereas front-side complex formation is negligible in the F À + CH 3 Cl reaction. Following some pioneering work, 125 our study 48 provided the first quantitative dynamical characterization of frontside complex formation in S N 2 reactions.…”

Imaging the dynamics of ion–molecule reactions

“…The latter reaction is exothermic and it was found that for temperatures as low as 200 K the proton transfer and the S N 2 reaction have nearly equal reaction probability. 163 The proton transfer and halogen abstraction channels have only recently started to receive theoretical attention. 157,[163][164][165] For example, it has been shown for F À + CH 3 I that the proton transfer channel features the same X-H bonded pre-reaction complex (C s minimum in Fig.…”