Roundabout Mechanism of Ion-Molecule Nucleophilic Substitution Reactions

Roundabout (RA) is an important indirect mechanism for gas-phase X^- + CH₃Y → XCH₃ + Y^- S_N2 reactions at a high collision energy. It refers to the rotation of the CH₃-group by half or multiple circles upon the collision of incoming nucleophiles before substitution takes place. The RA mechanism was first discovered in the Cl^- + CH₃I S_N2 reaction to explain the energy transfer observed in crossed molecular beam imaging experiments in 2008. Since then, the RA mechanism and its variants have been observed not only in multiple C-centered S_N2 reactions, but also in N-centered S_N2 reactions, proton transfer reactions, and elimination reactions. This work reviewed recent studies on the RA mechanism and summarized the characteristics of RA mechanisms in terms of variant types, product energy partitioning, and product velocity scattering angle distribution. RA mechanisms usually happen at small impact parameters and tend to couple with other mechanisms at relatively low collision energy, and the available energy of roundabout trajectories is primarily partitioned to internal energy. Factors that affect the importance of the RA mechanism were analyzed, including the type of leaving group and nucleophile, collision energy, and microsolvation. A massive leaving group and relatively high collision energy are prerequisite for the occurrence of the roundabout mechanism. Interestingly, when reacting with CH₃I, the importance of RA mechanisms follows an order of Cl^- > HO^- > F^-, and such a nucleophile dependence was attributed to the difference in proton affinity and size of the nucleophile.