Rotational Mode Specificity in the F– + CH3I(v = 0, JK) SN2 and Proton-Transfer Reactions

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Vol. 124; no. 43; pp. 8943 - 8948
• Main Authors: Papp, Paszkál, Czakó, Gábor
• Format: Journal Article
• Language: English
• Published: American Chemical Society 29.10.2020
• Subjects: A: Kinetics, Dynamics, Photochemistry, and Excited States
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/acs.jpca.0c08043

Quasiclassical trajectory computations are performed for the F– + CH3I­(v = 0, JK) → I– + CH3F (SN2) and HF + CH2I– (proton-transfer) reactions considering initial rotational states characterized by J = {0, 2, 4, 6, 8, 12, and 16} and K = {0 and J} in the 1–30 kcal/mol collision energy (E coll) range. Tumbling rotation (K = 0) counteracts orientation effects, thereby hindering the SN2 reactivity by about 15% for J = 16 in the 1–15 kcal/mol E coll range and has a negligible effect on proton transfer. Spinning about the C–I bond (K = J), which is 21 times faster than tumbling, makes the reactions more direct, inhibiting the SN2 reactivity by 25% in some cases, whereas significantly enhancing the proton-transfer channel by a factor of 2 at E coll = 15 kcal/mol due to the fact that the spinning-induced centrifugal force hinders complex formation by breaking H-bonds and activates C–H bond cleavage, thereby promoting proton abstraction on the expense of substitution. At higher E coll, as the reactions become more direct, the rotational effects are diminishing.