Theoretical Studies On The Reactivity Of Solvated Electrons In Aqueous Solution

Excess electrons in the aqueous solution can relax through solvation to form hydrated electrons.As an important reducing agent for aqueous reactions,hydrated electrons frequently appear in various radiation,catalysis,organic and biochemical reactions,and often exhibit extraordinary mechanism and activity based on their unique electronic structure and relative stability.In the research for half a century,a large number of experimental and theoretical studies have revealed the solvation structure,ground energy level,and excitation spectrum of hydrated electrons,and gradually accumulated reliable research experience in related systems.But the mechanism adopted by the hydrated electrons in reactions still needs to be further in-depth.They may generate many short but important intermediate states,unique and competing reaction pathways,and interactions with complex and randomly changing aqueous environments.In this work,we mainly use the DFT-based ab initio AIMD simulations to study the ultra-fast reaction dynamics mechanisms of excess electrons in different hydration stages in radiation,catalysis and biochemical systems.The main conclusions and innovations are briefly described as follows:(1)Molecular Dynamics Study on Hydrated Dielectron and Induced Spontaneous Hydrogen Evolution:The mechanism of hydrated electrons participating in the radiation reaction of the aqueous phase is a historical basic research topic.Among them,it is still lack of directly experimental evidence for the existence of an intermediate state which is named hydrated dielectron and involved in the aqueous phase hydrogen evolution induced by a pair of hydrated electrons.What's worse,the related theoretical studies are also controversial.In order to explore the possible forms of hydrated delectron and the role they play in hydrogen evolution by radiation,we used AIMD simulations based on hyper-GGA functional to obtain two different solvated forms of double excess electrons in aqueous solutions:closed-shell single state uni-caged hydrated dielectron and bi-caged hydrated electron pair.After comparing with the classic structure of hydrated electron,we found that hydrated dielectron have the similar characteristics of s-type spherical symmetric structure to single hydrated electron,but its electron distribution is more diffuse,and the coordination number is also slightly higher.The hydrated electron pair behaves like a pair of solvent-separated hydrated electrons,and each of them has a complete cavity structure and almost completely separated electron distribution,but it still exhibits some slight antiferromagnetic coupling characteristics.Through the calculations of thermodynamic integral,we also confirmed the stability of the hydrated dielectron structure and the feasibility of preparing through the recombination of hydrated electrons,which is in line with the expectations of previous experimental studies.We have discovered the spontaneous hydrogen evolution of pure water in many simulation trajectories,and analyze the irreplaceable role of hydrated dielectron among this reaction.The spontaneous hydrogen evolution mechanism of two-step proton transfer and forming the hydride anion intermediate is obtained.Our simulation conclusions can not only provide a new idea for the interpretation of various water radiolysis reactions,but also serve as an explanation of important side reaction pathways in most photocatalytic reactions,laying a foundation for subsequent studies on the catalytic mechanism including hydrated electrons or hydrated dielectron.(2)Molecular Dynamics Study on the Mechanism of Carbon Dioxide Reduction and Charge Transfer Induced by Hydrated Electrona:Experimental studies have found that hydrated electrons appear in the photocatalytic reduction of carbon dioxide,but the mechanism details have not yet been systematically explained.In order to explain the important role of excess electrons in different hydration states in these processes,we conducted a systematic study by means AIMD simulation.Through these simulations,we have not only obtained reasonable one/two-electron reduction products of carbon dioxide,but also discovered the details of ultrafast dynamics and electronic states that have not been noticed.We found the appropriate adjustments of the molecular and hydration structure have an important impact on the electron or proton transfer.We have also summarized and explained that the choice of charge transfer mechanisms is according to the target molecular orbital in the reduction reaction including hydrated electrons.These conclusions can help us better understand the reactivity of hydrated electrons caused by their electronic structure and energy characteristics,and provide a new theoretical basis for the exploration of related liquid or interface phase catalytic mechanisms.(3)Molecular Dynamics Study on the Protonation-Modulated Excess Electron Localization in Histidine Aqueous Solutions:Excess electron is also a major cause of radiation damage to life molecules,and the research on proteins or amino acids in this respect is slightly weaker than that of bases.Taking histidine in aqueous solution as a representative,we studied the localization dynamics of the un-hydrated excess electron in diffuse state to the specific groups of histidine.Since the imidazole group of the histidine side chain can change its protonation with the acidity of aqueous environment,we have discovered two different mechanisms for the localization of excess electron:the rapid localization dominated by the side-chain protonated imidazole group under slightly acidic conditions,and the slow localization dominated by the main-chain deprotonated carboxyl group under neutral conditions.We also analyzed the causes of these mechanisms based on the energy levels and compositions of the frontier molecular orbitals.This research allows us to fully realize the important influence of the structural changes of the amino acid main chain or side chain groups in the adsorption of excess electrons,and it also provides a new idea for regulating radiation damage or repair mechanisms through environmental factors.(4)Molecular Dynamics Study on Solvated Electron Catalytic Dissociation of Cyclobutane Pyrimidine Dimer:Excess electrons can also participate in the repair of important ultraviolet radiation damage products:cyclobutane pyrimidine dimer(CPD).However,the one-electron two-step catalytic mechanism CPD repairing has been accepted by many experiments and theoretical simulations.Recently,a research have proposed a new two-electron[2+2]synchronous dissociation mechanism for it,but its feasibility has not been actually verified.We reproduced the one-electron two-step catalytic mechanism of CPD dissociation by means of AIMD simulation,and supplemented it with electron-related dynamics details.But the simulation based on double excess electrons denies the possibility of two-electron mechanism.Based on the actual situation,we subsequently corrected and perfected the problems in this mechanism,and based on the symmetry characteristics of the frontier molecular orbitals in the core reaction zone,explained the actual role of the single and double excess electrons in the different steps of dissociation.This will provide valuable experience for exploring the actual reaction mechanism through theoretical calculations or simulations.In short,we mainly used ab initio molecular dynamics simulation to study different hydrated electrons participating in a variety of ultrafast chemical reactions,and discovered unique reaction mechanisms and important intermediates.These research advances will provide a reference for future research on the hydrated electron reaction mechanisms,and lays the foundation for its application in the fields of radiation,catalysis,and biochemistry.