Competition of F/OH-Induced SN2 and Proton-Transfer Reactions with Increased Solvation

Competition between Elimination and Substitution for Ambident Nucleophiles CN- and Iodoethane Reactions in Gaseous and Aqueous Medium

Nucleophilic substitution (SN2) and elimination (E2) reactions between ambident nucleophiles have long been considered as typical reactions in organic chemistry, and exploring the competition between the two reactions is of great importance in chemical synthesis. As a nucleophile, CN- can use its C and N atoms as the reactive centers to undergo E2 and SN2 reactions, but related research is currently limited. This study uses the CCSD(T)/pp/t//MP2/ECP/d electronic structure method to perform detailed investigations on the potential energy profiles for SN2 and E2 reactions between CN- and CH3CH2I in gaseous and aqueous media. The potential energy profiles reveal that the energy barriers for SN2 and E2 reactions with the C atom as the reactive center are consistently lower than those with the N atom, indicating that the C atom has a stronger nucleophilic ability and stronger basicity. Furthermore, the potential energy profiles in both gas and aqueous environments show that the barriers of SN2 reactions are lower than those for E2 reactions with both C and N as the attacking atom. By using the frontier molecular orbital and activation strain models to explain the interesting phenomenon, the transition from the gas phase to solution was investigated, specifically in the gas-microsolvation-water transition. The results show that water molecules reduce the nucleophilicity and basicity of CN-, while strain energy (ΔEstrain) causes a greater increase in the energy barrier for E2 reactions. This study provides new insights and perspectives on the understanding of CN- as a nucleophile in SN2 reactions and serves as theoretical guidance for organic synthesis.

Spectroscopic Study of the Br- +CH3 I→I- +CH3 Br SN 2 Reaction

Mass spectrometry and anion photoelectron spectroscopy have been used to study the gas-phaseS N 2 ${{{\rm S}}_{{\rm N}}2}$ reaction involvingB r - ${{{\rm B}{\rm r}}^{-}}$ andC H 3 I ${{{\rm C}{\rm H}}_{3}{\rm I}}$ . The anion photoelectron spectra associated with the reaction intermediates of thisS N 2 ${{{\rm S}}_{{\rm N}}2}$ reaction are presented. High-level CCSD(T) calculations have been utilised to investigate the reaction intermediates that may form as a result of theS N 2 ${{{\rm S}}_{{\rm N}}2}$ reaction along various different reaction pathways, including back-side attack and front-side attack. In addition, simulated vertical detachment energies of each reaction intermediate have been calculated to rationalise the photoelectron spectra.

Origin of the Microsolvation Effect on the Central Barriers of SN2 Reactions

We have quantitatively analyzed the microsolvation effect on the central barriers of microsolvated bimolecular nucleophilic substitution (S_N2) reactions by means of a two-step energy decomposition procedure. According to the first energy decompositions, an obvious increase in the central barrier for a microsolvated S_N2 reaction against its unsolvated counterpart can be mainly ascribed to the fact that the interaction between the solute and the conjunct solvent becomes less attractive from the reactant complex to the transition state. On the basis of the second energy decompositions with symmetry-adapted perturbation theory, this less attractive interaction in the transition state is primarily due to the interplay of the changes in the electrostatic, exchange, and induction components. However, the contribution of the change for the dispersion component is relatively small. A distinct linear correlation has also been observed between the changes of the total interaction energies and those of the corresponding electrostatic components for the microsolvated S_N2 reactions studied in this work. Moreover, the two-step energy decomposition procedure employed in this work is expected to be extensively applied to the gas phase reactions mediated by molecules or clusters.

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.

Free-Energy Landscape of the SN2 Reaction CH3Br + Cl- → CH3Cl + Br- in Different Liquid Environments

This work describes in detail the reaction path of the well-known S_N2 reaction CH₃Br + Cl^- → CH₃Cl + Br^-, whose reaction rate has a huge variation with the solvent in the gas phase and in protic and aprotic liquid environments. We employed the ASEC-FEG method to optimize for minima (reactants and products) and saddle points (transition states) in the in-solution free-energy hypersurface. The method takes atomistic details of the solvent into account. A polarizable continuum model (PCM) has also been employed for comparison. The most perceptive structural changes are noted in aqueous solution by using the ASEC-FEG approach. The activation energies in all solvents, estimated by means of free-energy perturbation calculations, are in good agreement with the experimental data. The total solute-solvent hydrogen bonds play an important role in the increased barrier height observed in water and are therefore crucial to explain the huge decrease in the kinetic constant. It is also found that the hydration shell around the ions breaks itself spontaneously to accommodate the molecule, thus forming minimum energy complexes.

Proton transfer-induced competing product channels of microsolvated Y-(H2O)n + CH3I (Y = F, Cl, Br, I) reactions

The potential energy profiles of three proton transfer-involved product channels for the reactions of Y^-(H₂O)_1,2 + CH₃I (Y = F, Cl, Br, I) were characterized using the B97-1/ECP/d method. These three channels include the (1) PT_CH3 product channel that transfers a proton from methyl to nucleophile, (2) HO^--induced nucleophilic substitution (HO^--S_N2) product channel, and (3) oxide ion substitution (OIS) product channel that gives CH₃O^- and HY products. The reaction enthalpies and barrier heights follow the order OIS > PT_CH3 > HO^--S_N2 > Y^--S_N2, and thus HO^--S_N2 can compete with the most favored Y^--S_N2 product channel under singly-/doubly-hydrated conditions, while the PT_CH3 channel only occurs under high collision energy and the OIS channel is the least probable. All product channels share the same pre-reaction complex, Y^-(H₂O)_n-CH₃I, in the entrance of the potential energy profile, signifying the importance of the pre-reaction complex. For HO^-/Y^--S_N2 channels, we considered front-side attack, back-side attack, and halogen-bonded complex mechanisms. Incremental hydration increases the barriers of both HO^-/Y^--S_N2 channels as well as their barrier difference, implying that the HO^--S_N2 channel becomes less important when further hydrated. Varying the nucleophile Y^- from F^- to I^- also increases the barrier heights and barrier difference, which correlates with the proton affinity of the nucleophiles. Energy decomposition analyses show that both the orbital interaction energy and structural deformation energy of the transition states determine the S_N2 barrier change trend with incremental hydration and varying Y^-. In brief, this work computes the comprehensive potential energy surfaces of the HO^--S_N2 and PT_CH3 channels and shows how proton transfer affects the microsolvated Y^-(H₂O)_1,2 + CH₃I reaction by competing with the traditional Y^--S_N2 channel.

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.

Investigating the role of halogen-bonded complexes in microsolvated Y−(H2O)n + CH3I SN2 reactions

A halogen-bonded complex pathway is computed for Y⁻(H₂O)_n + CH₃I (Y = HO, F, Cl, Br, and I) ion–molecule nucleophilic substitution reactions and is compared with back-side and front-side attack pathways.

Imaging Reaction Dynamics of F-(H2O) and Cl-(H2O) with CH3I

The dynamics of microhydrated nucleophilic substitution reactions have been studied using crossed beam velocity map imaging experiments and quasiclassical trajectory simulations at different collision energies between 0.3 and 2.6 eV. For F^–(H₂O) reacting with CH₃I, a small fraction of hydrated product ions I^–(H₂O) is observed at low collision energies. This product, as well as the dominant I^–, is formed predominantly through indirect reaction mechanisms. In contrast, a much smaller indirect fraction is determined for the unsolvated reaction. At the largest studied collision energies, the solvated reaction is found to also occur via a direct rebound mechanism. The measured product angular distributions exhibit an overall good agreement with the simulated angular distributions. Besides nucleophilic substitution, also ligand exchange reactions forming F^–(CH₃I) and, at high collision energies, proton transfer reactions are detected. The differential scattering images reveal that the Cl^–(H₂O) + CH₃I reaction also proceeds predominantly via indirect reaction mechanisms.

High-Level-Optimized Stationary Points for the F-(H2O) + CH3I System: Proposing a New Water-Induced Double-Inversion Pathway

We report 29 stationary points for the F^-(H₂O) + CH₃I reaction obtained by using the high-level explicitly correlated CCSD(T)-F12b method with the aug-cc-pVDZ basis set for the determination of the benchmark structures and frequencies and the aug-cc-pVQZ basis for energy computations. The stationary points characterize the monohydrated F^-- and OH^--induced Walden-inversion pathways and, for the first time, the front-side attack and F^--induced double-inversion mechanisms leading to CH₃F with retention as well as the novel H₂O-induced double-inversion retention pathway producing CH₃OH. Hydration effectively increases the relative energies of the stationary points, but the monohydrated inversion pathways are still barrierless, whereas the front-side attack and double-inversion barrier heights are around 30 and 20 kcal/mol, respectively.