Ab Initio Molecular Dynamics Approach To Investigate The Reaction Mechanisms Of The F^-+NH₂I S_N2 Reaction

An ab initio molecular dynamics（AIMD）approach was employed to study the reaction mechanisms of F^-+NH₂I at the N center.The bimolecular nucleophilic substitution reaction is a very important class of reactions in chemistry.Compared with the study on the mechanisms of the S_N2 reactions at carbon,the mechanisms of S_N2reactions at nitrogen atom are less understood.So,in this paper,F^-+NH₂I system is selected as the research target,and ab initio molecular dynamics approach is used to explore the types of S_N2 reaction mechanism in gas phase.In our study,ab initio molecular dynamics method is adopted to simulate the reaction path of two body collision,this method neither assumes any reaction mechanism,nor constructs potential energy surface.Instead,random initial conditions of the reaction are given,including random initial conditions,random initial velocities and random initial orientation.Firstly,the initial random position of the nucleus is fixed,and then Schrodinger equation of the electron is solved by density functional theory to obtain the potential energy of the system.According to the first derivative of the potential energy,the fore on the nucleus can be calculated,under the action of the force field,the nucleus moves to a new position,and the above process has been repeated until the end of reaction.We modified the quantum-molecular-dynamics module in the NWChem program to deal with a two-body collision problem to perform the AIMD trajectory calculations with the DFT/M06-2x/aug-cc-p VDZ（LANL2DZ-ECP）level of theory.We found 9 types of reaction in the target system,among which 6 types of bimolecular nucleophilic substitution reaction mechanism were found.Not only do we find one-transition-state mechanisms,these mechanisms also occur at carbon.We also found two-transition-states mechanisms and three-transition-states mechanisms:proton-abstraction induced inversion and Walden-inversion（PAII&WI）mechanism;proton-abstraction roundabout and backside-attack reaction（PAR&BAR）mechanism;front-side attack and backside-attack reaction（FSA&BAR）mechanism and proton-abstraction induced inversion and proton-abstraction roundabout and backside-attack reaction（PAII&PAR&BAR）mechanism.Among these complex mechanisms,PAII&WI and PAR&BAR mechanisms have been discovered at carbon and nitrogen.However,the FSA&BAR mechanism of two transition states and PAII&PAR&BAR mechanism of three transition states are first discovered.In addition,our results show that the S_N2 reactivity decreases gradually with the increase of collision energy at 0.03e V,0.1 e V and 0.5 e V,while the reactivity of proton-abstraction reaction mechanism is increases.At the high collision energy of 0.5 e V,the reactivity is mainly determined by proton-abstraction reaction mechanism.For the S_N2 reactivity,the two-transition-states mechanisms,especially the PAII&WI mechanism contributed the most to the reactivity of S_N2 reaction（52%,52%and 56%）at three collision energy,followed by one-transition-state mechanisms（37%,35%and 22%）,and the least contribution was the complex mechanism with three transition states（11%,13%and 22%）.Anyhow,for the S_N2 reaction F^-+NH₂I at the N center,two-transition-state mechanism and three-transition-states especially PAII&WI mechanism has the greatest contribution to the S_N2 reactivity,even more than one-transition-state.Therefor the PAII&WI mechanism plays a crucial role in the S_N2 reaction.These results indicate that there still many areas to be explored about the S_N2reactions,especially for the reactions system in which the heavy iodine atom as the leaving group,Iodine atom always acts as an observer in the proton-abstraction induced inversion while participating in the formation of new bonds and the breaking of old bonds in the Walden-inversion process.As for these complex mechanism in S_N2reaction,more research and study should be done in theory and experiment.In addition,it is very interesting to study the effect of polar solvents on the reaction mechanisms.For example,it is possible to investigate whether the composite mechanisms also exist in polarized solvents environment,and whether the solvent has effect on the molecular geometry and energy at the stationary points.We plan to explore the S_N2 reaction mechanisms on the title reaction in aqueous in the future.