F−(H2O)+CH3I ligand exchange reaction dynamics

Reaction dynamics of the methoxy anion CH3O- with methyl iodide CH3I

Studying larger nucleophiles in bimolecular nucleophilic substitution (SN2) reactions bridges the gap from simple model systems to those relevant to organic chemistry. Therefore, we investigated the reaction dynamics between the methoxy anion (CH3O-) and iodomethane (CH3I) in our crossed-beam setup combined with velocity map imaging at the four collision energies 0.4, 0.7, 1.2, and 1.6 eV. We find the two ionic products I- and CH2I-, which can be attributed to the SN2 and proton transfer channels, respectively. The proton transfer channel progresses in a previously observed fashion from indirect to direct scattering with increasing collision energy. Interestingly, the SN2 channel exhibits direct dynamics already at low collision energies. Both the direct stripping, leading to forward scattering, and the direct rebound mechanism, leading to backward scattering into high angles, are observed.

Shapeshifting Nucleophiles HO-(NH3)n React with Methyl Chloride

The microsolvated anions HO-(NH3)n were found to induce new nucleophile NH2-(H2O)(NH3)n-1 via intramolecular proton transfer. Hence, the ion-molecule nucleophilic substitution (SN2) reaction between CH3Cl and these shapeshifting nucleophiles lead to both the HO- path and NH2- path, meaning that the respective attacking nucleophile is HO- or NH2-. The CCSD(T) level of calculation was performed to characterize the potential energy surfaces. Calculations indicate that the HO- species are lower in energy than the NH2- species, and the SN2 reaction barriers are lower for the HO- path than the NH2--path. Incremental solvation increases the barrier for both paths. Comparison between HO-(NH3)n and HOO-(NH3)n confirmed the existence of an α-effect under microsolvated conditions. Comparison between HO-(NH3)n and HO-(H2O)n indicated that the more polarized H2O stabilizes the nucleophiles more than NH3, and thus, the hydrated systems have higher SN2 reaction barriers. The aforementioned barrier changes can be explained by the differential stabilization of the nucleophile and HOMO levels upon solvation, thus affecting the HOMO-LUMO interaction between the nucleophile and substrate. For the same kind of nucleophilic attacking atom, O or N, the reaction barrier has a good linear correlation with the HOMO level of the nucleophiles. Hence, the HOMO level or the binding energy of microsolvated nucleophiles is a good indicator to evaluate the order of barrier heights. This work expands our understanding of the microsolvation effect on prototype SN2 reactions beyond the water solvent.

Imaging Frontside and Backside Attack in Radical Ion-Molecule Reactive Scattering

We report on the reactive scattering of methyl iodide, CH₃I, with atomic oxygen anions O^-. This radical ion-molecule reaction can produce different ionic products depending on the angle of attack of the nucleophile O^- on the target molecule. We present results on the backside and frontside attack of O^- on CH₃I, which can lead to I^- and IO^- products, respectively. We combine crossed-beam velocity map imaging with quantum chemical calculations to unravel the chemical reaction dynamics. Energy-dependent scattering experiments in the range of 0.3-2.0 eV relative collision energy revealed that three different reaction pathways can lead to I^- products, making it the predominant observed product. Backside attack occurs via a hydrogen-bonded complex with observed indirect, forward, and sideways scattered iodide products. Halide abstraction via frontside attack produces IO^-, which mainly shows isotropic and backward scattered products at low energies. IO^- is observed to dissociate further to I^- + O at a certain energy threshold and favors more direct dynamics at higher collision energies.

Dynamical Effects of SN2 Reactivity Suppression by Microsolvation: Dynamics Simulations of the F-(H2O) + CH3I Reaction on a 21-Dimensional Potential Energy Surface

A comparison of atomistic dynamics between microsolvated and unsolvated reactions can expose the precise role of solvent molecules and thus provide deep insight into how solvation influences chemical reactions. Here we developed the first full-dimensional analytical potential energy surface of the F-(H2O) + CH3I reaction, which facilitates the efficient dynamics simulations on a quantitatively accurate level. The computed SN2 reactivity suppression ratio of the monosolvated F-(H2O) + CH3I reaction relative to the unsolvated F- + CH3I reaction as a function of collision energy first increases and then decreases steadily, forming an inverted-V shape, due to the combined dynamical effects of interaction time, steric hindrance, and collision-induced dehydration. Moreover, further analysis reveals that the steric effect of the F-(H2O) + CH3I reaction resulting from the single water molecule is manifested mainly in dragging the F- anion away from the central C atom, rather than shielding F- from C. Our study shows there is great potential in rigorously studying the role of the solvent in more complicated reactions.

Investigating the competing E2 and SN2 mechanisms for the microsolvated HO-(H2O)n=0-4 + CH3CH2X (X = Cl, Br, I) reactions

We characterized the anti-E2, syn-E2, inv-S_N2, and ret-S_N2 reaction channels for the reaction of microsolvated HO^-(H₂O)_n anions with CH₃CH₂X (X = Cl, Br, I), using the CCSD(T)/PP/t//MP2/ECP/d level method, to understand how a solvent influences the competing E2 and S_N2 reactions. The calculated sequence of barrier for the four channels is ret-S_N2 > syn-E2 > anti-E2 > inv-S_N2. The barrier heights increase with incremental hydration as the system transfers from the gas phase to microsolvation, and to bulk solvation (using the PCM implicit solvent model). As the degree of hydration n increases, good correlations have been found between barrier heights and several thermodynamic, geometric and charge parameters, including the reaction enthalpy, proton/ethyl-cation affinity of the hydrated nucleophile, geometric looseness (%L^‡) and asymmetry (%AS^‡) and charge asymmetry (Δq(X-O)) of the transition structures. Under a molecular orbital scheme, the HOMOs of nucleophiles are stabilized by stepwise hydration, explaining the rise in the barriers. Considering the effect of the leaving group, the barrier heights exhibit linear correlation with the halogen electronegativity and H-acidity of substrate CH₃CH₂X. In terms of E2/S_N2 competition, the barrier difference, , first increases then decreases as the number of explicit water molecules increases, under both microsolvation and bulk solvation conditions, but the inv-S_N2 pathway is always favored over the anti-E2 pathway. Energy decomposition analysis attributes the increase of barrier difference to the greater geometric distortion in the anti-E2 transition structure.

Single Solvent Molecules Induce Dual Nucleophiles in Gas-Phase Ion-Molecule Nucleophilic Substitution Reactions

Direct dynamics simulation of singly hydrated peroxide ion reacting with CH₃Cl reveals a new product channel that forms CH₃OH + Cl^- + HOOH, besides the traditional channel that forms CH₃OOH + Cl^- + H₂O. This finding shows that singly hydrated peroxide ion behaves as a dual nucleophile through proton transfer between HOO^-(H₂O) and HO^-(HOOH). Trajectory analysis attributes the occurrence of the thermodynamically and kinetically unfavored HO^--induced pathway to the entrance channel dynamics, where extensive proton transfer occurs within the deep well of the prereaction complex. This study represents the first example of a single solvent molecule altering the nucleophile in a gas-phase ion-molecule nucleophilic substitution reaction, in addition to reducing the reactivity and affecting the dynamics, signifying the importance of dynamical effects of solvent molecules.

Fifty years of nucleophilic substitution in the gas phase

Bimolecular nucleophilic substitution ( S N 2 ) reactions have become a model system for the investigation of structure-reactivity relationships, stereochemistry, solvent influences, and detailed atomistic dynamics. In this review, the progress during five decades of experimental and theoretical research on gas phase S N 2 reactions is discussed. Many advancements of the employed methods have led to a tremendous increase in our understanding of the properties and the dynamics of these reactions. For reactions involving six atoms a quantitative agreement of the differential reactive scattering cross sections has already been achieved, in the future it is expected that even larger polyatomic reactions systems become tractable. Furthermore, studies with higher precision, improved reactant control, and a more accurate theoretical treatment of quantum effects are envisioned.