Vibrational mode-specific dynamics of the F− + CH3CH2Cl multi-channel reaction

Abstract

We investigate the mode-specific dynamics of the ground-state, C–Cl stretching (v₁₀), CH₂ wagging (v₇), sym-CH₂ stretching (v₁), and sym-CH₃ stretching (v₃) excited F⁻ + CH₃CH₂Cl(v_k = 0, 1) [k = 10, 7, 1, 3] → Cl⁻ + CH₃CH₂F (S_N2), HF + CH₃CHCl⁻, FH⋯Cl⁻ + C₂H₄, and Cl⁻ + HF + C₂H₄ (E2) reactions using a full-dimensional high-level analytical global potential energy surface and the quasi-classical trajectory method. Excitation of the C–Cl stretching, CH₂ stretching, and CH₂/CH₃ stretching modes enhances the S_N2, proton abstraction, and FH⋯Cl⁻ and E2 channels, respectively. Anti-E2 dominates over syn-E2 (kinetic anti-E2 preference) and the thermodynamically-favored S_N2 (wider reactive anti-E2 attack angle range). The direct (a) S_N2, (b) proton abstraction, (c) FH⋯Cl⁻ + C₂H₄, (d) syn-E2, and (e) anti-E2 channels proceed with (a) back-side/backward, (b) isotropic/forward, (c) side-on/forward, (d) front-side/forward, and (e) back-side/forward attack/scattering, respectively. The HF products are vibrationally cold, especially for proton abstraction, and their rotational excitation increases for proton abstraction, anti-E2, and syn-E2, in order. Product internal-energy and mode-specific vibrational distributions show that CH₃CH₂F is internally hot with significant C–F stretching and CH₂ wagging excitations, whereas C₂H₄ is colder. One-dimensional Gaussian binning technique is proved to solve the normal mode analysis failure caused by methyl internal rotation.