Direct Dynamics Simulations Of Substitution Reaction Between Fluoride And Haloalkanes And Chloramines

Bimolecular nucleophilic substitution?S_N2?reaction,as a basic and important chemical reaction mechanism,has been widely applied in organic synthe sis and drug design.Chemical dynamics simulations can better explore the microscopic characteristics of the S_N2 reaction.Herein,influences of solvent environment,substituents,central atoms and collision energy on S_N2 reaction dynamics are uncovered by direct chemical dynamics simulations.We revealed microscopic dynamic behaviors of the S_N2 reaction,such as the possibility of reaction,reaction mechanism,product percentage,etc.The research results are meaningful for exploring reactions dynamics in organic and biochemistry.The dynamics of F-?H₂O?_0-3 + CH₃I reactions are uncovered in detail by using direct chemical dynamics simulations.Direct rebound and stripping and indirect atomic-level mechanisms are observed.The direct substitution mechanism,which is important without microsolvation?n = 0?.However,with the addition of solvent molecules,an indirect scattering is found to prevail due to form complexes at the entrance channel.Especially,collision energy has an important role on indirect mechanisms,which occurs dominantly by forming hydrated F-?H₂O?–HCH₂I and F-?H₂O?–CH₃I pre-reaction complexes at low energies,but proceeds through their water-free counterparts F-–HCH₂I and F-–CH₃I at higher energies.As established in experiments,solvation suppresses the reactivity,whereas we find that this depression is remarkably frustrated upon raising the energy given that collisioninduced dehydration essentially diminishes the water block for reactive collisions.The reaction dynamics show propensity for the direct three-body dissociation channel F-?H₂O?_n + CH₃I ? CH₃F + I-+ nH₂O after passing the reaction's dynamical bottleneck.The water molecule leaves the reactive system before traversing the postreaction region of the PES,where water transfer toward the product species occurs.This provides an insight into the very interesting finding of strongly suppressed formation of energetically favored solvated products for almost all S_N2 reactions under microsolvation.In addition,with upon increasing hydration,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.The dissertation explores the effect of substituents on the S_N2 reaction by simulating the F-?CH₃OH?_0-2 + CH₃CH₂ Br reaction.Introducing bulky substituents at the halogenated center carbon atom??-carbon?is supposed to frustrate Walden inversion and instead promote an E2 process.Here,we unravel how individual solvent molecules may affect underlying S_N2/E2 atomistic dynamics.Reaction dynamics simulations reproduce experimental findings at near thermal energies and show that the E2 mechanism dominates over S_N2 for solvent-free reaction.This is energetically quite unexpected and results from dynamical effects.Adding one solvating methanol molecule introduces strikingly distinct dynamical behaviors that largely promote the S_N2 reaction,a feature which attributes to a differential solute-solvent interaction at the central barrier that more strongly stabilizes the transition state for substitution.Upon further solvation,this enhanced stabilization of the S_N2 mechanism becomes more pronounced,concomitant with drastic suppression of the E2 route.This work highlights the interplay between energetics and dynamics in determining mechanistic selectivity and provides insight into the impact of solvent molecules on a general transition from elimination to substitution for chemical reactions proceeding from gas-to solution-phase environments.In addition,the prototypical E2 elimination and S_N2 substitution reactions show unexpected dynamical behaviors in mechanistic evolution driven by s olvation and collision activation.Considering the steric effects,the gas-phase selectivity favors an E2 pathway barely dependent of collision energies.Remarkably,base solvation steers the reaction in an effective way towards substitution at a near-thermal energy,whereas the governing high-energy events retain elimination.Chemical dynamics simulations reproduce experimental findings and uncover a crucial solute-solvent coupling in determining such competing processes.Interestingly,collision activation can tune the underlying atomistic dynamics essentially in reactant entrance channel and causes a mechanism shift.These features for the competing E2/S_N2 dynamics provide unique insight into reaction selectivity for organic synthesis.The central atom also has an important influence on the S_N2 reaction dynamics.The dissertation uncovered S_N2@N reaction F-+ NH₂Cl and show strikingly distinct features from a S_N2@C congener F-+ CH₃ Cl.Indirect scattering is found to prevail,which proceeds predominantly through a hydrogen-bonded F-–HNHCl complex.This unexpected finding of a pronounced contribution of indirect reaction dynamics,even at a high collision energy,is in strong contrast to a general evolution from indirect to direct dynamics with enhanced energy that characterizes S_N2@C.This result suggests that the relative importance of different atomic-level mechanisms may depend essentially on the interaction potential of reactive encounters and the coupling between inter-and intramolecular modes of the prereaction complex.For F-+ NH₂Cl the proton transfer pathway is less competitive than S_N2.A remarkable finding is that the more favorable energetics for NH₂Cl proton transfer,as compared to that for CH₃Cl,does not manifest itself in the reaction dynamics.