Effects of microsolvation on a SN2 reaction: indirect atomistic dynamics and weakened suppression of reactivity

Competitive dynamics of elimination and substitution reactions modulated using nucleophiles and leaving groups

The competition between base-induced elimination (E2) and bimolecular nucleophilic substitution (SN2) reactions and their intrinsic reactivity are hot issues in organic chemistry research. To investigate the influence factors of E2/SN2 channel selectivity, the HO- + CH3CH2Br reaction was performed utilizing direct dynamics simulations to unravel how the nucleophile and leaving group modulate the microscopic mechanisms of the X- (X = F and HO) + CH3CH2Y (Y = Cl and Br) reactions. Our simulations showed a significant increase in the direct mechanism branching ratio from 0.41 to 0.62 when the nucleophile was changed from F- to HO-. This mechanism shift was driven by the entrance channel complex's geometric configuration and the ion-molecular intermediate's lifetime. The disappearance of hydrogen-bonded complexes suppressed prolonged interactions of the prereaction complex, with more than half of the trajectories separating into products directly after the first collision. When the leaving group was changed from Cl to Br, the anti-E2 channel still dominated for the HO- + CH3CH2Br reaction, although its decreased proportion indicated that SN2 was more competitive. This result was attributed to the decrease in the bmax value in the HO- + CH3CH2Br reaction, which diminished the role of direct stripping mechanism at large collision parameters and ultimately decreased the probability of the anti-E2 reaction. This study underscores the impact of nucleophiles and leaving groups on the dynamics of E2/SN2 competition and its microscopic mechanisms, providing valuable insights into reaction selectivity in complex chemical environments and systems.

Atomistic dynamics of elimination and substitution driven by entrance channel

E2 elimination and SN2 substitution reactions are of central importance in preparative organic synthesis due to their stereospecificity. Herein, atomistic dynamics of a prototype reaction of ethyl chloride with hydroxide ion are uncovered that show strikingly distinct features from the case with fluoride anion. Chemical dynamics simulations reproduce the experimental reaction rate and reveal that the E2 proceeding through a direct elimination mechanism dominates over SN2 for the hydroxide ion reaction. This unexpected finding of a pronounced contribution of direct reaction dynamics, even at a near-thermal energy, is in strong contrast to the complex-mediated indirect mechanism for the fluoride case that characterizes the low-energy ion-molecule reactions. The entrance channel structures are found to be crucial and the differences are attributed to subtle changes in the hydrogen-bonding interaction of the approaching reactants. This effect presents in E2/SN2 reactions of different bases and alkyl halides and might play a role in complex chemical networks and environments.

Benchmark Ab Initio Mapping of the F- + CH2ClI SN2 and Proton-Abstraction Reactions

The experimental and theoretical studies of gas-phase SN2 reactions have significantly broadened our understanding of the mechanisms governing even the simplest chemical processes. These investigations have not only advanced our knowledge of reaction pathways but also provided critical insights into the fundamental dynamics of chemical systems. Nevertheless, in the case of the prototypical X- + CH3Y → Y- + CH3X [X, Y = F, Cl, Br, and I] SN2 reactions, the effect of the additional halogenation of CH3Y has not been thoroughly explored. Thus, here, we perform the first high-level ab initio characterization of the F- + CH2ClI SN2 and proton-abstraction reactions utilizing the explicitly-correlated CCSD(T)-F12b method. Two possible SN2 channels leading to the Cl- + CH2FI and I- + CH2FCl products are distinguished, in which we investigate four different pathways of back-side attack Walden inversion, front-side attack, double inversion, and halogen-bonded complex formation. In order to obtain the benchmark energies of the geometries of the stationary points, determined at the CCSD(T)-F12b/aug-cc-pVTZ level of theory, additional computations are carried out considering the basis set effects, post-CCSD(T) correlations, and core corrections. Using the benchmark data, we assess the accuracy of the MP2, DF-MP2, MP2-F12, and DF-MP2-F12 methods as well. By comparing the present F- + CH2ClI system with the corresponding F- + CH3Y [Y = Cl and I] reactions, this study demonstrates that further halogenation of CH3Y significantly promotes the corresponding proton-abstraction and SN2 retention channels as well as the halogen-bonded complex formation, and as a consequence, the traditional back-side attack Walden-inversion mechanism becomes less pronounced.

Roundabout Mechanism of Ion-Molecule Nucleophilic Substitution Reactions

Roundabout (RA) is an important indirect mechanism for gas-phase X- + CH3Y → XCH3 + Y- SN2 reactions at a high collision energy. It refers to the rotation of the CH3-group by half or multiple circles upon the collision of incoming nucleophiles before substitution takes place. The RA mechanism was first discovered in the Cl- + CH3I SN2 reaction to explain the energy transfer observed in crossed molecular beam imaging experiments in 2008. Since then, the RA mechanism and its variants have been observed not only in multiple C-centered SN2 reactions, but also in N-centered SN2 reactions, proton transfer reactions, and elimination reactions. This work reviewed recent studies on the RA mechanism and summarized the characteristics of RA mechanisms in terms of variant types, product energy partitioning, and product velocity scattering angle distribution. RA mechanisms usually happen at small impact parameters and tend to couple with other mechanisms at relatively low collision energy, and the available energy of roundabout trajectories is primarily partitioned to internal energy. Factors that affect the importance of the RA mechanism were analyzed, including the type of leaving group and nucleophile, collision energy, and microsolvation. A massive leaving group and relatively high collision energy are prerequisite for the occurrence of the roundabout mechanism. Interestingly, when reacting with CH3I, the importance of RA mechanisms follows an order of Cl- > HO- > F-, and such a nucleophile dependence was attributed to the difference in proton affinity and size of the nucleophile.

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.

Competitive dynamics of E2 and SN2 reaction driven by collision energy and leaving group

The competition between E2 and SN2 reactions is essential in organic chemistry. In this paper, the reaction mechanism of F- + CH3CH2Cl is investigated utilizing direct dynamics simulations, and unravel how the collision energy (Ecoll) and the leaving group affect the competition between SN2 and E2 in the F- + CH3CH2Y (Y = Cl and Br) reactions. Simulation results for F- + CH3CH2Cl reaction show that the anti-E2 channel is dominant, but with the increase of Ecoll from 0.04 to 1.9 eV the branching ratio of the anti-E2 pathway significantly decreases by 21%, and the SN2 pathway becomes more important. A transition from indirect to direct reaction has been revealed when Ecoll is increased from 0.04 to 1.90 eV. At lower Ecoll, a large ratio of indirect events occurs via a long-lived hydrogen-bonded complex, and as the collision energy is increased, the lifetimes of the hydrogen-bonded complexes are shortened, due to an initial faster relative velocity. The simulation results of F- + CH3CH2Cl are further compared with the F- + CH3CH2Br reaction at Ecoll of 0.04 eV. Changing the leaving group from Cl to Br drastically suppresses the indirect events of anti-E2 with a branching ratio decreasing from 0.46 to 0.36 due to the mass effect, and promotes direct rebound mechanism resulting from a looser transition state geometry caused by varied electronegativity.

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.

Microsolvated Ion-Molecule SN2 Reactions with Dual Nucleophiles Induced by Solvent Molecules

Singly-hydrated HOO - anion was found to induce alternative nucleophile HO - via proton transfer from water molecule as react with CH 3 Cl recently. To investigate the generality of this effect, the competition between the solvent-induced HO - -S N 2 pathway and the normal HOO - -S N 2 pathway is studied for the microsolvated HOO - (H 2 O) n=1,2,3 + CH 3 X (X = F, Cl, Br, I) reaction by quantum chemistry calculation. Incremental hydration increases the barrier heights of both pathways and enlarges the barrier difference between them, which favors the HOO - -S N 2 pathway. Interestingly, the barrier difference is insensitive to the leaving group. Calculation shows the water induced HO - -S N 2 pathway is highly suppressed as the degree of hydration increases beyond two. The differential barrier under incremental hydration can be explained by solvent molecules stabilizing the HOMO level of HO - (HOOH)(H 2 O) n-1 nucleophile more than that of HOO - (H 2 O) n nucleophile. Comparison between these HO - -nucleophiles and HOO - -nucleophiles suggests that α-effect exists. Activation strain analysis attributes the barrier differences to the stronger distortion of the TS of HO - -S N 2 pathway than the counterparts of HOO - -S N 2 pathway. This work adds our understanding of the role of individual solvent molecules to induce new nucleophiles of the fundamental organic reaction.

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.

Effect of microsolvation on the mode specificity of the OH˙(H2O) + HCl reaction

The present study investigates the mode specificity in the microsolvated OH˙(H₂O) + HCl reaction using on-the-fly direct dynamics simulation. To the best of our knowledge, this is the first study which aims to gain insights into the effect of microsolvation on the mode selectivity. Our investigation reveals that, similar to the gas phase OH˙ + HCl reaction, the microsolvated reaction is also predominantly affected by the vibrational excitation of the HCl mode, whereas the OH vibrational mode behaves as a spectator. Interestingly, in contrast to the behavior of the bare reaction, the integral cross section at the ground state of the microsolvated reaction decreases with an increase in translational energy. However, for the vibrational excited states, the reactivity of the microsolvated reaction is found to be higher than that of the bare reaction within the selected range of translational energies.

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.

Dynamics of Cl-(H2O) + CH3I Substitution Reaction: The Influences of Solvent and Nucleophile

The study of microsolvation provides a deeper understanding of solvent effects on reaction dynamics. Here, the properties of the S_N2 reaction of hydrated chloride with methyl iodide are investigated by direct dynamics simulations, and how the solute-solvent interactions and the basicity of nucleophiles can profoundly affect the atomic level dynamics is discussed in detail. The results show that the direct-rebound mechanism dominates the substitution reaction, and the roundabout mechanism, which prevails in the indirect unsolvated counterpart reaction, still accounts for a high proportion of the indirect mechanisms. The involvement of a solvent water molecule does not significantly reduce the cross section and rate constant compared to the unhydrated reaction at high collision energy. By varying solvated Cl^- to F^-, the dominant mechanisms are totally different and in contrast, the dynamics of water does not show much difference, and the departure of H₂O tends to occur prior to the substitution reaction because of the facile breakage of the hydrogen bond at high collision energy.

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.

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

The potential energy profiles of F/OH-induced nucleophilic substitution (S_N2) and proton-transfer (PT) channels evolving with solvation for reactions of F^-(H₂O) _n=1-2 + CH₃I were characterized using B3LYP/ECP/d method. The hydrogen-bonded F^-(H₂O) _n---HCH₂I prereaction complex at the entrance of potential energy surface (PES) has a significant role on the reaction dynamics for each channel. Among the above three channels, the F-S_N2 channel is the most preferred and OH-S_N2 could be competitive. In contrast, the PT channel will occur at much higher collision energy. Importantly, for each channel, the central barrier is gradually increased with the addition of water molecules. This phenomenon indicates that the reactivity will decrease with degrees of solvation and this has been confirmed by experiment and direct dynamics simulations. Moreover, compared with the previous trajectory simulations, a non-IRC behavior has been uncovered. The water delivering process from fluorine to iodine side as illustrated on PES is barely observed, and instead, the reaction tends to dehydrate before passing through the S_N2 barrier and proceeds with the less hydrated pathway in order to weaken the steric effect. The work presented here shows the comprehensive potential energy surfaces and structures information on the F-S_N2, PT, and OH-S_N2 channels, and predict their competitive relationship, which would be helpful for better understanding the dynamics behavior of the title and analogous reactions.

Efficient synthesis of organic thioacetates in water

Thioacetates as precursors of thiols are interesting starting points for synthesizing other organosulfur compounds. Herein, we propose a simple, efficient and fast method to obtain organic thioacetates using water as a solvent. Taking into account the great attention that has been paid toward environmentally friendly synthetic procedures in the past decades, we prove the role and the strength of the thioacetate anion as a nucleophile for nucleophilic displacement reactions in an aqueous medium. The reactions were carried out under pH control, to prevent the decomposition of the mesylate starting materials, using potassium carbonate as a safe and mild base. A simple work up allows products to be obtained with excellent yield and acceptable purity.

Dynamic exit-channel pathways of the microsolvated HOO-(H2O) + CH3Cl SN2 reaction: Reaction mechanisms at the atomic level from direct chemical dynamics simulations

Microsolvated bimolecular nucleophilic substitution (S_N2) reaction of monohydrated hydrogen peroxide anion [HOO^-(H₂O)] with methyl chloride (CH₃Cl) has been investigated with direct chemical dynamics simulations at the M06-2X/6-31+G(d,p) level of theory. Dynamic exit-channel pathways and corresponding reaction mechanisms at the atomic level are revealed in detail. Accordingly, a product distribution of 0.85:0.15 is obtained for Cl^-:Cl^-(H₂O), which is consistent with a previous experiment [D. L. Thomsen et al. J. Am. Chem. Soc. 135, 15508 (2013)]. Compared with the HOO^- + CH₃Cl S_N2 reaction, indirect dynamic reaction mechanisms are enhanced by microsolvation for the HOO^-(H₂O) + CH₃Cl S_N2 reaction. On the basis of our simulations, further crossed molecular beam imaging experiments are highly suggested for the S_N2 reactions of HOO^- + CH₃Cl and HOO^-(H₂O) + CH₃Cl.

Steric Effects of Solvent Molecules on SN2 Substitution Dynamics

Influences of solvent molecules on S_N2 reaction dynamics of microsolvated F^-(H₂O)_n with CH₃I, for n = 0-3, are uncovered by direct chemical dynamics simulations. The direct substitution mechanism, which is important without microsolvation, is quenched dramatically upon increasing hydration. The water molecules tend to force reactive encounters to proceed through the prereaction collision complex leading to indirect reaction. In contrast to F^-(H₂O), reaction with higher hydrated ions shows a strong propensity for ion desolvation in the entrance channel, diminishing steric hindrance for nucleophilic attack. Thus, nucleophilic substitution avoids the potential energy barrier with all of the solvent molecules intact and instead occurs through the less solvated barrier, which is energetically unexpected because the former barrier has a lower energy. The work presented here reveals a trade-off between reaction energetics and steric effects, with the latter found to be crucial in understanding how hydration influences microsolvated S_N2 dynamics.

Imaging the dynamics of ion–molecule reactions

A range of ion–molecule reactions have been studied in the last years using the crossed-beam ion imaging technique, from charge transfer and proton transfer to nucleophilic substitution and elimination.