Identification of Atomic-Level Mechanisms for Gas-Phase X^– + CH₃Y S_N2 Reactions by Combined Experiments and Simulations

Abstract

For the traditional model of gas-phase X(-) + CH3Y SN2 reactions, C3v ion-dipole pre- and postreaction complexes X(-)---CH3Y and XCH3---Y(-), separated by a central barrier, are formed. Statistical intramolecular dynamics are assumed for these complexes, so that their unimolecular rate constants are given by RRKM theory. Both previous simulations and experiments have shown that the dynamics of these complexes are not statistical and of interest is how these nonstatistical dynamics affect the SN2 rate constant. This work also found there was a transition from an indirect, nonstatistical, complex forming mechanism, to a direct mechanism, as either the vibrational and/or relative translational energy of the reactants was increased. The current Account reviews recent collaborative studies involving molecular beam ion-imaging experiments and direct (on-the-fly) dynamics simulations of the SN2 reactions for which Cl(-), F(-), and OH(-) react with CH3I. Also considered are reactions of the microsolvated anions OH(-)(H2O) and OH(-)(H2O)2 with CH3I. These studies have provided a detailed understanding of the atomistic mechanisms for these SN2 reactions. Overall, the atomistic dynamics for the Cl(-) + CH3I SN2 reaction follows those found in previous studies. The reaction is indirect, complex forming at low reactant collision energies, and then there is a transition to direct reaction between 0.2 and 0.4 eV. The direct reaction may occur by rebound mechanism, in which the ClCH3 product rebounds backward from the I(-) product or a stripping mechanism in which Cl(-) strips CH3 from the I atom and scatters in the forward direction. A similar indirect to direct mechanistic transition was observed in previous work for the Cl(-) + CH3Cl and Cl(-) + CH3Br SN2 reactions. At the high collision energy of 1.9 eV, a new indirect mechanism, called the roundabout, was discovered. For the F(-) + CH3I reaction, there is not a transition from indirect to direct reaction as Erel is increased. The indirect mechanism, with prereaction complex formation, is important at all the Erel investigated, contributing up ∼60% of the reaction. The remaining direct reaction occurs by the rebound and stripping mechanisms. Though the potential energy curve for the OH(-) + CH3I reaction is similar to that for F(-) + CH3I, the two reactions have different dynamics. They are akin, in that for both there is not a transition from an indirect to direct reaction. However, for F(-) + CH3I indirect reaction dominates at all Erel, but it is less important for OH(-) + CH3I and becomes negligible as Erel is increased. Stripping is a minor channel for F(-) + CH3I, but accounts for more than 60% of the OH(-) + CH3I reaction at high Erel. Adding one or two H2O molecules to OH(-) alters the reaction dynamics from that for unsolvated OH(-). Adding one H2O molecule enhances indirect reaction at low Erel, and changes the reaction mechanism from primarily stripping to rebound at high Erel. With two H2O molecules the dynamics is indirect and isotropic at all collision energies.

Oxidation of Half-Lantern Pt2(II,II) Compounds by Halocarbons. Evidence of Dioxygen Insertion into a Pt(III)–CH3 Bond

“…Similar profiles were observed in related cases modeled by DFT, as in the bimolecular S N 2 reaction X − + CH 3 Y → XCH 3 + Y − . 71 Thus, the first step consists of the interaction of the Pt 2 (II,II) precursor acting as a nucleophile with the haloalkane. The formed adduct results from the stabilizing interaction of the Pt 2 -centered highest occupied molecular orbital of the dinuclear complex with the electrophillic C atom of the halogenated substrate.…”

Accurate ab initio potential energy surface, thermochemistry, and dynamics of the F− + CH3F SN2 and proton-abstraction reactions

“…However, recent theoretical and experimental investigations showed that the dynamics is not so simple. [3][4][5][6][7][8][9][10][11][12] Besides the direct rebound mechanism described above, direct stripping (X − approaches the side of CH 3 Y and strips off CH 3 ), front-side attack (direct replacement of Y − without inversion), and indirect mechanisms (complex formations, roundabout, 6 and barrier recrossing) can occur. Moreover, our recent reaction dynamics computations revealed a new double-inversion mechanism for the F − + CH 3 Cl S N 2 reaction.…”

Revealing a double-inversion mechanism for the F−+CH3Cl SN2 reaction

“…Due to the high barrier of the former, the S N 2 reactions are known to proceed via Walden inversion at low collision energies (E coll ) and the front-side attack pathway may open at higher E coll . Note that one may distinguish between direct rebound and stripping, as well as indirect (iondipole and/or hydrogen-bonded complex formation, roundabout and barrier recrossing) mechanisms 14 , but in the present study we consider these as variants of the back-side attack inversion mechanism.…”