Roundabout\nMechanism of Ion–Molecule Nucleophilic\nSubstitution Reactions

Roundabout (RA) is an important indirect mechanism for\ngas-phase\nX<sup>–</sup> + CH<sub>3</sub>Y → XCH<sub>3</sub> +\nY<sup>–</sup> S<sub>N</sub>2 reactions at a high collision\nenergy. It refers to the rotation of the CH<sub>3</sub>-group by half\nor multiple circles upon the collision of incoming nucleophiles before\n substitution takes place. The RA mechanism was first discovered in\nthe Cl<sup>–</sup> + CH<sub>3</sub>I S<sub>N</sub>2 reaction\nto explain the energy transfer observed in crossed molecular beam\nimaging experiments in 2008. Since then, the RA mechanism and its\nvariants have been observed not only in multiple C-centered S<sub>N</sub>2 reactions, but also in N-centered S<sub>N</sub>2 reactions,\nproton transfer reactions, and elimination reactions. This work reviewed\n recent studies on the RA mechanism and summarized the characteristics\nof RA mechanisms in terms of variant types, product energy partitioning,\nand product velocity scattering angle distribution. RA mechanisms\nusually happen at small impact parameters and tend to couple with\nother mechanisms at relatively low collision energy, and the available\nenergy of roundabout trajectories is primarily partitioned to internal\nenergy. Factors that affect the importance of the RA mechanism were\nanalyzed, including the type of leaving group and nucleophile, collision\nenergy, and microsolvation. A massive leaving group and relatively\nhigh collision energy are prerequisite for the occurrence of the roundabout\nmechanism. Interestingly, when reacting with CH<sub>3</sub>I, the\nimportance of RA mechanisms follows an order of Cl<sup>–</sup> > HO<sup>–</sup> > F<sup>–</sup>, and such a\nnucleophile\ndependence was attributed to the difference in proton affinity and\nsize of the nucleophile.