Microsolvation – a process in which reactants bind to a limited number of solvent molecules – provides a powerful means to understand solvent effects, including reduced reactivity, altered reaction dynamics, and enhanced α-effects, in the transition from gas-phase to condensed-phase reactions. In this work, we use direct dynamics simulations to show, for the first time, that a single solvent molecule can alter the reactive nucleophile in a bimolecular nucleophilic substitution reaction HOO-(H2O) + CH3Cl and affect its detailed dynamics. The computed stable structure of the singly-hydrated α-nucleophile is HO-(HOOH), but experiments suggest that HOO- is the active nucleophile rather than HO-. Our trajectory simulations primarily result in the HOO--SN2 pathway to form CH3OOH + Cl- + H2O, but reveal the existence of a HO--SN2 pathway to form CH3OH + Cl- + HOOH; these results clearly demonstrate that HO-(HOOH) can serve as the attacking nucleophile. The occurrence of HO--SN2 products is attributed to the reaction dynamics at the entrance channel. The mechanism distribution shows that indirect mechanisms involving pre-reaction complexes are dominant and mechanisms involving post-reaction complexes also occur. The trajectories are trapped in the pre-reaction complex well for long lifetimes, during which extensive proton exchange occurs between (H2O)HOO----CH3---Cl and (HOOH)HO----CH3---Cl pre-reaction complexes. Such interaction allows formation of both the CH3OOH products as well as the thermodynamically and kinetically unfavored CH3OH products. In addition, the simulations indicate that addition of a single water molecule reduces the reaction rate constant by one order of magnitude, in agreement with experiment. This work expands our understanding of solvation effects and reveals that a single solvent molecule can directly participate in the reaction and induce a new product channel.

Direct dynamics simulation of microhydrated HOO- ion with CH3Cl

ACS Spring Meeting 2021

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