The reactivity of each channel nearly holds unchanged except for the perturbation between anti-E2 and inv-S N2. Varying the leaving group for a series of reactions F – + CH 3CH 2Y (Y = F, Cl, Br, and I) leads to monotonically decreasing barriers, which relates to the gradually looser TS structures following the order F > Cl > Br > I. Negative activation energies (−16.9, −16.0, and −4.9 kcal/mol, respectively) for inv-S N2, anti-E2, and syn-E2 indicate that inv-S N2 and anti-E2 mechanisms significantly prevail over the eclipsed elimination. A positive barrier of +19.2 kcal/mol for ret-S N2 shows the least feasibility to occur at room temperature. Importantly, the controversy concerning the reactant complex for anti-E2 and inv-S N2 paths has been clarified in the present work. Geometrical analysis confirmed synchronous E2-type transition states for the elimination of the current reaction, instead of nonconcerted processes through E1cb-like and E1-like mechanisms. Anti-E2, syn-E2, inv-, and ret-S N2 reaction channels for the gas-phase reaction of F – + CH 3CH 2I were characterized with a variety of electronic structure calculations.
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