Theoretical Computational Study On Photochemical Behavior Of Psychoactive Substance Ketamine In Water Environment

All sectors of society are paying more and more attention to new pollutants that are difficult to be degraded in the environment.Compared with traditional chemical contaminants,such pollutants are more harmful to the ground and humans.Psychoactive substances are an essential part of new pollutants,and their residues in the background are also frequently detected.Photochemical conversion is the primary degradation pathway of such substances in the ground.Therefore,studying the photochemical conversion mechanism of such substances in the aquatic environment is of great significance to the safety of the ecological environment.In surface water bodies,there is widespread soluble organic matter(DOM)that can promote the photodegradation of organic pollutants.Therefore,this paper selects ketamine(KET)and the common DOM analog fluorenone(FL),which have a high detection rate in the water environment,as the computational simulation objects,and quantum chemistry to study the photochemical behavior of KET in the water environment.The main research contents and calculation results are as follows:(1)The reaction mechanism in the ground state S₀ is based on density functional theory(DFT),and the theoretical calculation in the excited state is based on time-dependent density functional theory(TDDFT).After determining the practical,combined with the transition state theory,the mechanism of the photolysis reaction is studied.The results showed that the direct photolysis of KET could occur in both the S₁ state and the T₁ state.In the excited state,the two ring structures of the KET structure are folded,the conjugation of the whole molecule is reduced,and the photo-reactivity of KET is high.In the S₁ state,the photolysis reaction of KET is mainly the cleavage of the N atom connected to the cyclohexanone group and the methyl group.Finally,the direct photolysis product of KET,nor-ketamine(NK),is formed.Secondly,a part of the KET in the singlet excited state will cause a hydrolysis reaction due to the destruction of the cyclic structure of the cyclohexanone group.Although KET cannot directly get the T₁ state by absorbing surface light energy,it can reach the excited triplet state through the intersystem crossing of molecules in the S₁ state.KET in the T₁ state can be directly photo-dechlorinated.The theoretically calculated reaction path matches the experimental results.(2)Under the premise of using the same functional and basis set,calculate the reaction mechanism of indirect photolysis of KET when fluorenone(FL),an analog of dissolved organic matter(DOM),is present in the reaction system.Base and natural bond orbital theory,compares the charge number and electron spin density of each molecule in the system before and after the reaction and obtain the charge transfer during the reaction.The result can be drawn:FL can absorb sunlight to reach an excited state and then degrade KET through electron transfer sensitization.Comparing the structure of the optimized reactant and the complex simulated in the reaction process,it can be determined that the first step of indirect photolysis is the combination of FL and KET to form a stable ground state complex,which is then excited by absorbing light energy together.Simultaneously,the charge of the N atom in KET decreases during the degradation process,and the negative charge of the O atom of FL increases,and there are single electrons in both KET and FL.Therefore,it can be determined that electrons are transferred from KET to FL,and electron rearrangement occurs,thereby achieving the purpose of sensitizing degradation of KET.