Imaging dynamic fingerprints of competing E2 and SN2 reactions

First-principles mode-specific reaction dynamics

Controlling the outcome of chemical reactions by exciting specific vibrational and/or rotational modes of the reactants is one of the major goals of modern reaction dynamics studies. In the present Perspective, we focus on first-principles vibrational and rotational mode-specific dynamics computations on reactions of neutral and anionic systems beyond six atoms such as X + C2H6 [X = F, Cl, OH], HX + C2H5 [X = Br, I], OH- + CH3I, and F- + CH3CH2Cl. The dynamics simulations utilize high-level ab initio analytical potential energy surfaces and the quasi-classical trajectory method. Besides initial state specificity and the validity of the Polanyi rules, mode-specific vibrational-state assignment for polyatomic product species using normal-mode analysis and Gaussian binning is also discussed and compared with experiment.

High-level analytical potential-energy-surface-based dynamics of the OH- + CH3CH2Cl SN2 and E2 reactions in full (24) dimensions

We develop a coupled-cluster full-dimensional global potential energy surface (PES) for the OH- + CH3CH2Cl reactive system, using the Robosurfer program package, which automatically samples configurations along PES-based trajectories as well as performs ab initio computations with Molpro and fitting with the monomial symmetrization approach. The analytical PES accurately describes both the bimolecular nucleophilic substitution (SN2) and elimination (E2) channels leading to the Cl- + CH3CH2OH and Cl- + H2O + C2H4 products, respectively, and allows efficient quasi-classical trajectory (QCT) simulations. QCT computations on the new PES provide accurate statistically-converged integral and differential cross sections for the OH- + CH3CH2Cl reaction, revealing the competing dynamics and mechanisms of the SN2 and E2 (anti, syn, β-α transfer) channels as well as various additional pathways leading to induced inversion of the CH3CH2Cl reactant, H-exchange between the reactants, H2O⋯Cl- complex formation, and H2O + CH3CHCl- products via proton abstraction.

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.

Computational Insights into SN 2 and Proton Transfer Reactions of CH3 O- with NH2 Y and CH3 Y

Bimolecular nucleophilic substitution (SN 2) reactions have been extensively studied in both theory and experiment. While research on C-centered SN 2 reactions (SN 2@C) has been ongoing, SN 2 reactions at neutral nitrogen (SN 2@N) have received increased attention in recent years. To recommend methods for dynamics simulations, the comparison for the properties of the geometries, vibrational frequencies, and energies is done between MP2 and six DFT functional calculations and experimental data as well as the high-level CCSD(T) method for CH3 O- +NH2 Cl/CH3 Cl reactions. The relative energy diagrams at the M06 method for CH3 O- with CH3 Y/NH2 Y reactions (Y=F, Cl, Br, I) in the gas and solution phase are explored to investigate the effects of the leaving groups, different reaction centers, and solvents. We mainly focus on the computational of inv-SN 2 and proton transfer (PT) pathways. The PT channel in the gas phase is more competitive than the SN 2 channel for N-center reactions, while the opposite is observed for C-centered reactions. Solvation completely inhibits the PT channel, making SN 2 the dominant pathway. Our study provides new insight into the SN 2 reaction mechanisms and rich the novel reaction model in gas-phase organic chemistry.

Accurate fundamental invariant-neural network representation of ab initio potential energy surfaces

Highly accurate potential energy surfaces are critically important for chemical reaction dynamics. The large number of degrees of freedom and the intricate symmetry adaption pose a big challenge to accurately representing potential energy surfaces (PESs) for polyatomic reactions. Recently, our group has made substantial progress in this direction by developing the fundamental invariant-neural network (FI-NN) approach. Here, we review these advances, demonstrating that the FI-NN approach can represent highly accurate, global, full-dimensional PESs for reactive systems with even more than 10 atoms. These multi-channel reactions typically involve many intermediates, transition states, and products. The complexity and ruggedness of this potential energy landscape present even greater challenges for full-dimensional PES representation. These PESs exhibit a high level of complexity, molecular size, and accuracy of fit. Dynamics simulations based on these PESs have unveiled intriguing and novel reaction mechanisms, providing deep insights into the intricate dynamics involved in combustion, atmospheric, and organic chemistry.

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.

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.

SN 2 versus E2 Competition of Cyclic Ethers

We have quantum chemically studied the influence of ring strain on the competition between the two mechanistically different SN 2 and E2 pathways using a series of archetypal ethers as substrate in combination with a diverse set of Lewis bases (F- , Cl- , Br- , HO- , H3 CO- , HS- , H3 CS- ), using relativistic density functional theory at ZORA-OLYP/QZ4P. The ring strain in the substrate is systematically increased on going from a model acyclic ether to a 6- to 5- to 4- to 3-membered ether ring. We have found that the activation energy of the SN 2 pathway sharply decreases when the ring strain of the system is increased, thus on going from large to small cyclic ethers, the SN 2 reactivity increases. In contrast, the activation energy of the E2 pathway generally rises along this same series, that is, from large to small cyclic ethers. The opposing reactivity trends induce a mechanistic switch in the preferred reaction pathway for strong Lewis bases from E2, for large cyclic substrates, to SN 2, for small cyclic substrates. Weak Lewis bases are unable to overcome the higher intrinsic distortivity of the E2 pathway and, therefore, always favor the less distortive SN 2 reaction.

Effects of Methyl Substitution and Leaving Group on E2/SN2 Competition for Reactions of F- with RY (R = CH3, C2H5, iC3H7, tC4H9; Y = Cl, I)

The competition between base-induced elimination (E2) and bimolecular nucleophilic substitution (SN2) is of significant importance in organic chemistry and is influenced by many factors. The electronic structure calculations for the gas-phase reactions of F- + RY (R = CH3, C2H5, iC3H7, tC4H9, and Y = Cl, I) are executed at the MP2 level with aug-cc-pVDZ or ECP/d basis set to investigate the α-methyl substitution effect. The variation in barrier height, reaction enthalpy, and competition of SN2/E2 as a function of methyl-substitution and leaving group ability has been emphasized. And the nature of these rules has been explored. As the degree of methyl substitution on α-carbon increases, the E2 channel becomes more competitive and dominant with R varying from C2H5, iC3H7, to tC4H9. Energy decomposition analysis offers new insights into the competition between E2 and SN2 processes, which suggests that the drop in interaction energy with an increasing degree of substitution cannot compensate for the rapid growth of preparation energy, leading to a rapid increase in the SN2 energy barrier. By altering the leaving group from Cl to I, the barriers of both SN2 and E2 monotonically decrease, and, with the increased number of substituents, they reduce more dramatically, which is attributed to the looser transition state structures with the stronger leaving group ability. Interestingly, ∆E0‡ exhibits a positive linear correlation with reaction enthalpy (∆H) and halogen electronegativity. With the added number of substituents, the differences in ∆E0‡ and ∆H between Y = Cl and I likewise exhibit good linearity.

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.

Reaction mechanism conversion induced by the contest of nucleophile and leaving group

Direct dynamic simulations have been employed to investigate the OH^- + CH₃Cl reaction with the chosen B3LYP/aug-cc-pVDZ method. The calculated rate coefficient for the bimolecular nucleophilic substitution reaction (S_N2), 1.0 × 10^-9 cm³ mol^-1 s^-1 at 300 K, agrees well with the experimental result of (1.3-1.6) × 10^-9 cm³ mol^-1 s^-1. The simulations reveal that the majority of the S_N2 reactions are temporarily trapped in the hydrogen-bonded complex at E_coll = 0.89 kcal mol^-1. Importantly, the influences of the leaving group and nucleophile have been discussed by comparisons of X^- + CH₃Y (X = F, OH; Y = Cl, I) reactions. For the X = F^- reactions, the reaction probability of S_N2 increases along the increased leaving group ability Cl < I, suggesting that the thermodynamic factor plays a key role. The indirect mechanisms were found to be dominant for both reactions. In contrast, for X = OH^-, the fraction of S_N2 drops with the enhanced leaving group ability. In particular, a dramatic transition occurs for the dominant atomic reaction mechanisms, i.e., from complex-mediated indirect to direct, implying an interesting contest between the leaving group and the nucleophile and the importance of the dynamic factors, i.e., the dipole moment, steric hindrance, and electronegativity.

Unexpected steric hindrance failure in the gas phase F- + (CH3)3CI SN2 reaction

Base-induced elimination (E2) and bimolecular nucleophilic substitution (S_N2) reactions are of significant importance in physical organic chemistry. The textbook example of the retardation of S_N2 reactivity by bulky alkyl substitution is widely accepted based on the static analysis of molecular structure and steric environment. However, the direct dynamical evidence of the steric hindrance of S_N2 from experiment or theory remains rare. Here, we report an unprecedented full-dimensional (39-dimensional) machine learning-based potential energy surface for the 15-atom F⁻ + (CH₃)₃CI reaction, facilitating the reliable and efficient reaction dynamics simulations that can reproduce well the experimental outcomes and examine associated atomic-molecular level mechanisms. Moreover, we found surprisingly high “intrinsic” reactivity of S_N2 when the E2 pathway is completely blocked, indicating the reaction that intends to proceed via E2 transits to S_N2 instead, due to a shared pre-reaction minimum. This finding indicates that the competing factor of E2 but not the steric hindrance determines the small reactivity of S_N2 for the F⁻ + (CH₃)₃CI reaction. Our study provides new insight into the dynamical origin that determines the intrinsic reactivity in gas-phase organic chemistry.

Base-induced elimination (E2) and bimolecular nucleophilic substitution (SN2) are of significant importance in physical organic chemistry. Here, the authors show that the competing factor of E2 as opposed to steric hindrance determines the low reactivity of SN2 in the F⁻ + (CH₃)₃CI reaction.

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.

Vibrational mode-specific dynamics of the F- + CH3CH2Cl multi-channel reaction

We investigate the mode-specific dynamics of the ground-state, C-Cl stretching (v₁₀), CH₂ wagging (v₇), sym-CH₂ stretching (v₁), and sym-CH₃ stretching (v₃) excited F^- + CH₃CH₂Cl(v_k = 0, 1) [k = 10, 7, 1, 3] → Cl^- + CH₃CH₂F (S_N2), HF + CH₃CHCl^-, FH⋯Cl^- + C₂H₄, and Cl^- + HF + C₂H₄ (E2) reactions using a full-dimensional high-level analytical global potential energy surface and the quasi-classical trajectory method. Excitation of the C-Cl stretching, CH₂ stretching, and CH₂/CH₃ stretching modes enhances the S_N2, proton abstraction, and FH⋯Cl^- and E2 channels, respectively. Anti-E2 dominates over syn-E2 (kinetic anti-E2 preference) and the thermodynamically-favored S_N2 (wider reactive anti-E2 attack angle range). The direct (a) S_N2, (b) proton abstraction, (c) FH⋯Cl^- + C₂H₄, (d) syn-E2, and (e) anti-E2 channels proceed with (a) back-side/backward, (b) isotropic/forward, (c) side-on/forward, (d) front-side/forward, and (e) back-side/forward attack/scattering, respectively. The HF products are vibrationally cold, especially for proton abstraction, and their rotational excitation increases for proton abstraction, anti-E2, and syn-E2, in order. Product internal-energy and mode-specific vibrational distributions show that CH₃CH₂F is internally hot with significant C-F stretching and CH₂ wagging excitations, whereas C₂H₄ is colder. One-dimensional Gaussian binning technique is proved to solve the normal mode analysis failure caused by methyl internal rotation.

Does Steric Hindrance Actually Govern the Competition between Bimolecular Substitution and Elimination Reactions?

Bimolecular nucleophilic substitution (S_N2) and elimination (E2) reactions are prototypical examples of competing reaction mechanisms, with fundamental implications in modern chemical synthesis. Steric hindrance (SH) is often considered to be one of the dominant factors determining the most favorable reaction out of the S_N2 and E2 pathways. However, the picture provided by classical chemical intuition is inevitably grounded on poorly defined bases. In this work, we try to shed light on the aforementioned problem through the analysis and comparison of the evolution of the steric energy (E_ST), settled within the IQA scheme and experienced along both reaction mechanisms. For such a purpose, the substitution and elimination reactions of a collection of alkyl bromides (R-Br) with the hydroxide anion (OH^-) were studied in the gas phase at the M06-2X/aug-cc-pVDZ level of theory. The results show that, generally, E_ST recovers the appealing trends already anticipated by chemical intuition and organic chemistry, supporting the role that SH is classically claimed to play in the competition between S_N2 and E2 reactions.

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.

Solvent Effects on the Menshutkin Reaction

The Menshutkin reaction is a methyl transfer reaction relevant in fields ranging from biochemistry to chemical synthesis. In the present work, the energetics and solvent distributions for NH₃+MeCl and Pyr+MeBr reactions were investigated in explicit solvent (water, methanol, acetonitrile, benzene, cyclohexane) by means of reactive molecular dynamics simulations. For polar solvents (water, methanol, and acetonitrile) and benzene, strong to moderate catalytic effects for both reactions were found, whereas apolar and bulky cyclohexane interacts weakly with the solute and does not show pronounced barrier reduction. The calculated barrier heights for the Pyr+MeBr reaction in acetonitrile and cyclohexane are 23.2 and 28.1 kcal/mol compared with experimentally measured barriers of 22.5 and 27.6 kcal/mol, respectively. The solvent distributions change considerably between reactant and TS but comparatively little between TS and product conformations of the solute. As the system approaches the transition state, correlated solvent motions occur which destabilize the solvent-solvent interactions. This is required for the system to surmount the barrier. Finally, it is found that the average solvent-solvent interaction energies in the reactant, TS, and product state geometries are correlated with changes in the solvent structure around the solute.

How Solvation Influences the SN2 versus E2 Competition

We have quantum chemically investigated how solvation influences the competition between the S_N2 and E2 pathways of the model F^– + C₂H₅Cl reaction. The system is solvated in a stepwise manner by going from the gas phase, then via microsolvation of one to three explicit solvent molecules, then last to bulk solvation using relativistic density functional theory at (COSMO)-ZORA-OLYP/QZ4P. We explain how and why the mechanistic pathway of the system shifts from E2 in the gas phase to S_N2 upon strong solvation of the Lewis base (i.e., nucleophile/protophile). The E2 pathway is preferred under weak solvation of the system by dichloromethane, whereas a switch in reactivity from E2 to S_N2 is observed under strong solvation by water. Our activation strain and Kohn–Sham molecular orbital analyses reveal that solvation of the Lewis base has a significant impact on the strength of the Lewis base. We show how strong solvation furnishes a weaker Lewis base that is unable to overcome the high characteristic distortivity associated with the E2 pathway, and thus the S_N2 pathway becomes viable.

Uncovering an oxide ion substitution for the OH- + CH3F reaction

Theoretical investigations on chemical reactions allow us to understand the dynamics of the possible pathways and identify new unexpected routes. Here, we develop a global analytical potential energy surface (PES) for the OH⁻ + CH₃F reaction in order to perform high-level dynamics simulations. Besides bimolecular nucleophilic substitution (S_N2) and proton abstraction, our quasi-classical trajectory computations reveal a novel oxide ion substitution leading to the HF + CH₃O⁻ products. This exothermic reaction pathway occurs via the CH₃OH⋯F⁻ deep potential well of the S_N2 product channel as a result of a proton abstraction from the hydroxyl group by the fluoride ion. The present detailed dynamics study of the OH⁻ + CH₃F reaction focusing on the surprising oxide ion substitution demonstrates how incomplete our knowledge is of fundamental chemical reactions.

Reaction dynamics simulations on a high-level ab initio analytical potential energy surface reveal a novel oxide ion substitution channel for the OH⁻ + CH₃F reaction.

Atomistic dynamics of elimination and nucleophilic substitution disentangled for the F- + CH3CH2Cl reaction

Chemical reaction dynamics are studied to monitor and understand the concerted motion of several atoms while they rearrange from reactants to products. When the number of atoms involved increases, the number of pathways, transition states and product channels also increases and rapidly presents a challenge to experiment and theory. Here we disentangle the dynamics of the competition between bimolecular nucleophilic substitution (S_N2) and base-induced elimination (E2) in the polyatomic reaction F^- + CH₃CH₂Cl. We find quantitative agreement for the energy- and angle-differential reactive scattering cross-sections between ion-imaging experiments and quasi-classical trajectory simulations on a 21-dimensional potential energy hypersurface. The anti-E2 pathway is most important, but the S_N2 pathway becomes more relevant as the collision energy is increased. In both cases the reaction is dominated by direct dynamics. Our study presents atomic-level dynamics of a major benchmark reaction in physical organic chemistry, thereby pushing the number of atoms for detailed reaction dynamics studies to a size that allows applications in many areas of complex chemical networks and environments.

Chemical reactivity from an activation strain perspective

Chemical reactions are ubiquitous in the universe, they are at the core of life, and they are essential for industrial processes. The drive for a deep understanding of how something occurs, in this case, the mechanism of a chemical reaction and the factors controlling its reactivity, is intrinsically valuable and an innate quality of humans. The level of insight and degree of understanding afforded by computational chemistry cannot be understated. The activation strain model is one of the most powerful tools in our arsenal to obtain unparalleled insight into reactivity. The relative energy of interacting reactants is evaluated along a reaction energy profile and related to the rigidity of the reactants' molecular structure and the strength of the stabilizing interactions between the deformed reactants: ΔE(ζ) = ΔEstrain(ζ) + ΔEint(ζ). Owing to the connectedness between the activation strain model and Kohn-Sham molecular orbital theory, one is able to obtain a causal relationship between both the sterics and electronics of the reactants and their mutual reactivity. Only when this is accomplished one can eclipse the phenomenological explanations that are commonplace in the literature and textbooks and begin to rationally tune and optimize chemical transformations. We showcase how the activation strain model is the ideal tool to elucidate fundamental organic reactions, the activation of small molecules by metallylenes, and the cycloaddition reactivity of cyclic diene- and dipolarophiles.

A benchmark ab initio study of the complex potential energy surfaces of the OH- + CH3CH2Y [Y = F, Cl, Br, I] reactions

We provide the first benchmark characterization of the OH^- + CH₃CH₂Y [Y = F, Cl, Br, I] reactions utilizing the high-level explicitly-correlated CCSD(T)-F12b method with the aug-cc-pVnZ [n = 2(D), 3(T), 4(Q)] basis sets. We explore and analyze the stationary points of the elimination (E2) and substitution (S_N2) reactions, including anti-E2, syn-E2, back-side attack, front-side attack, and double inversion. In all cases, S_N2 is thermodynamically more preferred than E2. In the entrance channel of S_N2 a significant front-side complex formation is revealed, and in the product channel the global minimum of the title reactions is obtained at the hydrogen-bonded CH₃CH₂OHY^- complex. Similar to the OH^- + CH₃Y reactions, double inversion can proceed via a notably lower-energy pathway than front-side attack, moreover, for Y = I double inversion becomes barrier-less. For the transition state of the anti-E2, a prominent ZPE effect emerges, giving an opportunity for a kinetically more favored pathway than back-side attack. In addition to S_N2 and E2, other possible product channels are considered, and in most cases, the benchmark reaction enthalpies are in excellent agreement with the experimental data.

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.

First-Principles Reaction Dynamics beyond Six-Atom Systems

Moving beyond the six-atomic benchmark systems, we discuss the new age and future of first-principles reaction dynamics, which investigates complex, multichannel chemical reactions. We describe the methodology starting from the benchmark ab initio characterization of the stationary points, followed by full-dimensional potential energy surface (PES) developments and reaction dynamics computations. We highlight our composite ab initio approach providing benchmark stationary-point properties with subchemical accuracy, the Robosurfer program system enabling automatic PES development, and applications for the Cl + C2H6, F + C2H6, and OH- + CH3I post-six-atom reactions focusing on ab initio issues and their solutions as well as showing the excellent agreement between theory and experiment.

A Unified Framework for Understanding Nucleophilicity and Protophilicity in the SN 2/E2 Competition

The concepts of nucleophilicity and protophilicity are fundamental and ubiquitous in chemistry. A case in point is bimolecular nucleophilic substitution (SN 2) and base-induced elimination (E2). A Lewis base acting as a strong nucleophile is needed for SN 2 reactions, whereas a Lewis base acting as a strong protophile (i.e., base) is required for E2 reactions. A complicating factor is, however, the fact that a good nucleophile is often a strong protophile. Nevertheless, a sound, physical model that explains, in a transparent manner, when an electron-rich Lewis base acts as a protophile or a nucleophile, which is not just phenomenological, is currently lacking in the literature. To address this fundamental question, the potential energy surfaces of the SN 2 and E2 reactions of X- +C2 H5 Y model systems with X, Y = F, Cl, Br, I, and At, are explored by using relativistic density functional theory at ZORA-OLYP/TZ2P. These explorations have yielded a consistent overview of reactivity trends over a wide range in reactivity and pathways. Activation strain analyses of these reactions reveal the factors that determine the shape of the potential energy surfaces and hence govern the propensity of the Lewis base to act as a nucleophile or protophile. The concepts of "characteristic distortivity" and "transition state acidity" of a reaction are introduced, which have the potential to enable chemists to better understand and design reactions for synthesis.

Dynamics of a gas-phase SNAr reaction: non-concerted mechanism despite the Meisenheimer complex being a transition state

The commonly accepted mechanism of the nucleophilic aromatic substitution (SNAr) reaction has been found to be governed by the nature of the Meisenheimer structure on the potential energy surface. A stable Meisenheimer intermediate favors a stepwise mechanism, while a Meisenheimer transition state favors a concerted mechanism. Here, we show by using a detailed potential energy map (using the DFT and DLPNO-CCSD(T)/CBS methods) and ab initio classical trajectory simulations that the F- + C6H5NO2 SNAr reaction involves a Meisenheimer transition state and follows a stepwise mechanism in contrast to the expected concerted pathway. The stepwise mechanism observed in the trajectory simulations takes place by the formation of various ion-dipole and σ-complexes. While the majority of the trajectories follow the multi-step mechanism and avoid the minimum energy path, a considerable fraction exhibit a roaming atom mechanism where the F atom hovers around the phenyl ring before the formation of the products.

Quantitative Dynamics of the N2O + C2H2 → Oxadiazole Reaction: A Model for 1,3-Dipolar Cycloadditions

The reaction N₂O + C₂H₂ → oxadiazole has been considered as a prototype for 1,3-dipolar cycloadditions. Here, we report a comprehensive dynamical study of this important reaction on a full-dimensional potential energy surface, which is fitted to about 64 000 high-level ab initio data by a machine learning approach. Comprehensive dynamical simulations are carried out to provide quantitative chemical insight into its reaction dynamics. In addition to confirming the enhancement effect of the N₂O bending mode on the reactivity, intricate mode specificity effects of other vibrational modes in reactants are revealed for the first time. The asymmetric stretching mode of N₂O and the C-C-H bending mode of C₂H₂ show no effect. All remaining modes can enhance the reactivity. In particular, the vibrational excitation of the N₂O symmetric stretching mode shows similar enhancement effect on the title reaction, compared to its bending mode excitation. Detailed analysis reveals that the concerted mechanism dominates with the reactants propelled sufficiently close to each other to yield product. This study advances our understanding of the chemical dynamics of the title reaction.

Proton transfer dynamics modified by CH-stretching excitation

Gaining insight how specific rovibrational states influence reaction kinetics and dynamics is a fundamental goal of physical chemistry. Purely statistical approaches often fail to predict the influence of a specific state on the reaction outcome, evident in a great number of both experimental and theoretical studies. Most detailed insight in atomistic reaction mechanisms is achieved using accurate collision experiments and high level dynamics calculations. For ion-molecule reactions such experiments are scarce. Here we show the influence of symmetric CH-stretching vibration on the rate and dynamics of proton transfer in the reaction of F- + CH3I. We find a pronounced shift in the reaction dynamics for excited reactions from indirect to preferred direct dynamics at higher collision energy. Moreover, excited reactions occur at larger impact parameters. Finally, we compare vibrational excitation with collision energy and find that vibration is overall more efficient in promoting reactivity, which agrees with recent theoretical calculations.

Influence of Vibrational Excitation on the Reaction of F- with CH3I: Spectator Mode Behavior, Enhancement, and Suppression

Detailed insight into chemical reaction dynamics can be obtained by probing the effect of mode-specific vibrational excitation. Suppression or enhancement of reactivity is possible as is already known from the Polanyi rules. In the reaction F- + CH3I, we found vibrational enhancement, suppression, and spectator mode dynamics in the four different reaction channels. For this system we have probed the influence of symmetric CH-stretching vibration over a collision energy range of 0.7-2.3 eV. Proton transfer is significantly enhanced, while for the nucleophilic substitution channel the spectator mode dynamics at lower collision energies unexpectedly move toward enhancement at higher collision energies. In contrast, for two halide abstraction channels, forming FI- and FHI-, we found an overall suppression, which stems mainly from a suppression of the FHI- product. We compare these results to quasiclassical trajectory calculations and with the sudden vector projection model.

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.

Stable chaos and delayed onset of statisticality in unimolecular dissociation reactions

Statistical models provide a powerful and useful class of approximations for calculating reaction rates by bypassing the need for detailed, and often difficult, dynamical considerations. Such approaches invariably invoke specific assumptions about the extent of intramolecular vibrational energy flow in the system. However, the nature of the transition to the statistical regime as a function of the molecular parameters is far from being completely understood. Here, we use tools from nonlinear dynamics to study the transition to statisticality in a model unimolecular reaction by explicitly visualizing the high dimensional classical phase space. We identify generic features in the phase space involving the intersection of two or more independent anharmonic resonances and show that the presence of correlated, but chaotic, intramolecular dynamics near such junctions leads to nonstatisticality. Interestingly, akin to the stability of asteroids in the Solar System, molecules can stay protected from dissociation at the junctions for several picoseconds due to the phenomenon of stable chaos.

Benchmark ab initio and dynamical characterization of the stationary points of reactive atom + alkane and SN2 potential energy surfaces

We review composite ab initio and dynamical methods and their applications to characterize stationary points of atom/ion + molecule reactions.

Intramolecular vibrational energy redistribution and the quantum ergodicity transition: a phase space perspective

The onset of facile intramolecular vibrational energy flow can be related to features in the connected network of anharmonic resonances in the classical phase space.

Nucleophilic substitution vs elimination reaction of bisulfide ions with substituted methanes: exploration of chiral selectivity by stereodirectional first-principles dynamics and transition state theory

Control of molecular orientation is emerging as crucial for the characterization of the stereodynamics of kinetics processes beyond structural stereochemistry. The special role played in chiral discrimination phenomena has been particularly emphasized by Aquilanti and collaborators after their extensive probes of experimental control of molecular alignment and orientation. In this work, the manifestation of the Aquilanti mechanism has been demonstrated for the first time in first-principles molecular dynamics simulations: stationary points characterized on potential energy surfaces have been calculated for the study of chemical reactions occurring between the bisulfide anion HS^- and oriented prototypical chiral molecules CHFXY (where X = CH₃ or CN and Y = Cl or I). The important reaction channels are those corresponding to bimolecular nucleophilic substitution (S_N2) and to bimolecular elimination (E2): their relative role has been assessed and alternative pathways due to the mirror forms of the oriented chiral molecule are revealed by the different reactivity of the two enantiomers of CHFCNI in S_N2 reaction.

Multi-mass velocity-map imaging studies of photoinduced and electron-induced chemistry

Multi-mass velocity-map imaging (VMI) is becoming established as a promising method for probing the dynamics of a variety of gas-phase chemical processes. We provide an overview of velocity-map imaging and multi-mass velocity-map imaging techniques, highlighting examples in which these approaches have been used to provide mechanistic insights into a range of photoinduced and electron-induced chemical processes. Multi-mass detection capabilities have also led to the development of two new tools for the chemical dynamics toolbox, in the form of Coulomb-explosion imaging and covariance-map imaging. These allow details of molecular structure to be followed in real time over the course of a chemical reaction, offering the tantalising prospect of recording real-time 'molecular movies' of chemical dynamics. As these new methods become established within the reaction dynamics community, they promise new mechanistic insights into chemistry relevant to fields ranging from atmospheric chemistry and astrochemistry through to synthetic organic photochemistry and biology.

New Method To Extract Final-State Information of Polyatomic Reactions Based on Normal Mode Analysis

State-to-state reaction dynamics provides a comprehensive insight into reaction mechanisms of chemical reactions at the atomic level. A new scheme to extract final-state information based on normal mode analysis is proposed in this work. Different from the traditional scheme extracting the coordinates and momenta from the last step of each trajectory, they are taken in the new scheme from a specific step of each reactive trajectory within the last vibrational period of the product molecule by demanding the corresponding geometry of the step to have the minimum potential energy. Test calculations on the collisions between the atom H and the molecules H₂O, H₂S, and NH₃ show that the new scheme works much better than the traditional one. In addition, the new scheme is applied to calculate the vibrational state distribution of the product NH₂ in the reaction H + NH₃ → H₂ + NH₂.

Stretching vibration is a spectator in nucleophilic substitution

How chemical reactions are influenced by reactant vibrational excitation is a long-standing question at the core of chemical reaction dynamics. In reactions of polyatomic molecules, where the Polanyi rules are not directly applicable, certain vibrational modes can act as spectators. In nucleophilic substitution reactions, CH stretching vibrations have been considered to be such spectators. While this picture has been challenged by some theoretical studies, experimental insight has been lacking. We show that the nucleophilic substitution reaction of F^- with CH₃I is minimally influenced by an excitation of the symmetric CH stretching vibration. This contrasts with the strong vibrational enhancement of the proton transfer reaction measured in parallel. The spectator behavior of the stretching mode is supported by both quasi-classical trajectory simulations and the Sudden Vector Projection model.

Nucleophilic Substitution (SN 2): Dependence on Nucleophile, Leaving Group, Central Atom, Substituents, and Solvent

The reaction potential energy surface (PES), and thus the mechanism of bimolecular nucleophilic substitution (SN 2), depends profoundly on the nature of the nucleophile and leaving group, but also on the central, electrophilic atom, its substituents, as well as on the medium in which the reaction takes place. Here, we provide an overview of recent studies and demonstrate how changes in any one of the aforementioned factors affect the SN 2 mechanism. One of the most striking effects is the transition from a double-well to a single-well PES when the central atom is changed from a second-period (e. g. carbon) to a higher-period element (e.g, silicon, germanium). Variations in nucleophilicity, leaving group ability, and bulky substituents around a second-row element central atom can then be exploited to change the single-well PES back into a double-well. Reversely, these variations can also be used to produce a single-well PES for second-period elements, for example, a stable pentavalent carbon species.

Potential energy surface stationary points and dynamics of the F- + CH3I double inversion mechanism

Direct dynamics simulations were performed to study the S_N2 double inversion mechanism S_N2-DI, with retention of configuration, for the F^- + CH₃I reaction. Previous simulations identified a transition state (TS) structure, i.e. TS0, for the S_N2-DI mechanism, including a reaction path. However, intrinsic reaction coordinate (IRC) calculations from TS0 show it is a proton transfer (PT) TS connected to the F^-HCH₂I S_N2 pre-reaction complex and the FHCH₂I^- proton transfer post-reaction complex. Inclusion of TS0 in the S_N2-DI mechanism would thus involve non-IRC atomistic dynamics. Indeed, trajectories initiated at TS0, with random ensembles of energies as assumed by RRKM theory, preferentially form the S_N2-DI products and ∼70% follow the proposed S_N2-DI pathway from TS0 to the products. In addition, the Sudden Vector Projection (SVP) method was used to identify which CH₃I vibrational mode excitations promote access to TS0 and the S_N2-DI mechanism. Results of F^- + CH₃I simulations, with SVP specified mode excitations, are disappointing. With the CH₃ deformations of CH₃I excited, the S_N2 single inversion mechanism is the dominant pathway. If the CH stretch modes are also excited, proton transfer dominates the reaction. S_N2-DI occurs, but with a very small probability of ∼1%. The reasons behind these results are discussed.

Conservation of direct dynamics in sterically hindered SN2/E2 reactions

Nucleophilic substitution (S_N2) and base-induced elimination (E2), two indispensable reactions in organic synthesis, are commonly assumed to proceed under stereospecific conditions. Understanding the way in which the reactants pre-orient in these reactions, that is its stereodynamics, is essential in order to achieve a detailed atomistic picture and control over such processes. Using crossed beam velocity map imaging, we study the effect of steric hindrance in reactions of Cl^- and CN^- with increasingly methylated alkyl iodides by monitoring the product ion energy and scattering angle. For both attacking anions the rebound mechanism, indicative of a direct S_N2 pathway, is found to contribute to the reaction at high relative collision energies despite being increasingly hindered. An additional forward scattering mechanism, ascribed to a direct E2 reaction, also contributes at these energies. Inspection of the product energy distributions confirms the direct and fast character of both mechanisms as opposed to an indirect reaction mechanism which leads to statistical energy redistribution in the reaction complex. This work demonstrates that nonstatistical dynamics and energetics govern S_N2 and E2 pathways even in sterically hindered exchange reaction systems.

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.