Chemical dynamics simulations of gas-phase ion-molecule reactions: Investigating the micro-solvation effect

Date

2015-05

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Abstract

The study of microsolvated anions allows one to investigate the transition of ion-molecule reaction dynamics from gas-phase to solution. In this thesis direct dynamics simulations, with the B97-1/ECP/d theory, were performed to study the OH-(H2O)n=0,1 + CH3I reaction at an atomistic level and to investigate the role of microsolvation. Various product channels were observed. For the non-solvated OH- + CH3I reaction, the bimolecular nucleophilic substitution SN2 pathway dominates at low collision energy and the proton transfer pathway dominates at high collision energy. For mono-solvated OH-(H2O) + CH3I reaction, the SN2 pathway dominates at all collision energies. Adding a single H2O molecule already affects the dynamics. For example, the reaction rate is decreased by a factor of 2 and the reaction mechanisms become more complicated. The SN2 reactions occur by direct rebound and stripping mechanisms, and multiple indirect mechanisms. The pre-reaction complexes HO----HCH2I- and (H2O)HO----HCH2I- are important for the SN2 reactions, whereas the post-reaction complexes CH3OH---I- and (H2O)CH3OH---I- are not important. The simulations are in excellent agreement with experiments in aspect of rate constant, product branching ratio, and product energy partitioning. For the OH-(H2O) + CH3I SN2 reaction, equilibrium solvation by the H2O molecule is unimportant. The solvated product I-(H2O) is highly suppressed as compared to the unsolvated product I-.

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Keywords

Direct Dynamics, SN2, Microsolvation Effect, DFT

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