Systematic studies of microsolvation in the gas phase have enriched our knowledge of solvent effects. Here, the dynamics of a prototype SN2 reaction of a hydrated fluoride ion with methyl iodide is uncovered employing direct dynamics simulations that show strikingly distinct features from those determined for an unsolvated system. An indirect scattering is found to prevail, which occurs dominantly by forming hydrated F-(H2O)-HCH2I and F-(H2O)-CH3I pre-reaction complexes at low energies, but proceeds through their water-free counterparts at higher energies. This finding is in strong contrast to a general evolution from indirect to direct dynamics with enhancing energy for the unsolvated substitution reactions, and this discrepancy is understood by the substantial steric hindrance introduced by a water molecule. As established in experiments, solvation suppresses the reactivity, whereas we find that this depression is remarkably frustrated upon raising the energy given that collision-induced dehydration essentially diminishes the water block for reactive collisions. The present study sheds light on how solute-solvent interactions affect the underlying dynamics at a deeper atomic level, thereby promoting our understanding of the fundamental solvent effects on chemical reactions in solution.
Bibliographical noteFunding Information:
This work was supported by the National Natural Science Foundation of China (no. 21573052, 21403047, 51536002) and the Fundamental Research Funds for the Central Universities, China (AUGA5710012114, 5710012014). Support was also provided by the High Performance Computing Center (HPCC) at Texas Tech University, under the direction of Philip W. Smith. The authors wish to acknowledge important discussions with Professor W. L. Hase concerning the direct dynamics simulations.
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