Photoreaction Mechanism And Non-adiabatic Kinetic Studies Of Criegee Intermediates

Criegee intermediate(CI)is a very reactive ketone oxide produced during the ozonolysis of alkene.Since was first proposed by Rudolf Criegee as a kind of zwitterion in 1949,CI has been widely concerned by chemists.Due to the difficulty of formation,there are only a few experimental reports on the structure and absorption spectrum of CI.Most of the decomposition mechanism were proposed in the theoretical studies,while due to the limitations of calculation methods and computer performance,the ideal calculation accuracy can not be achieved in the early stage.In recent years,with the improvement of calculation methods and hardwares,increasing number of relevant theoretical works are reported.Theoretical and experimental studies show that the intermediate of Criegee is not a pure zwitterion,but also has some characteristics of diradical.The special electronic structure makes it react with H2O,SO2,NO2,CH4 or H2CO in the atmosphere or decompose in unimolecular reaction rapidly,among which the later one is always more important.At present,most of the understanding of CI unimolecular decomposition is focused on thermal reaction.It has been well known that,in the process of unimolecular thermal decomposition of the simplest Criegee intermediate(SCI,CH2OO),in which C-O-O cyclization is the first-step reaction,and then further dissociates into CO+H2O,CO2+H2 or HCO+·OH.Besides,the mechanisms of thermal unimolecular reactions may be greatly affected by the introductions of different substituents.In short,when substituted by saturated substituents,in which the hydrgen interacts with terminal oxygen,hydrogen transfer usually takes place as the first step;while when substituted by unsaturated substituents,electrocyclization is more likely to occur.Compared with the ground state,there are more unknowns about the photo-induced decomposition mode of CIs.On the one hand,the ultra-fast reaction rate increases the difficulty of experimental detection;on the other hand,the theoretical requirements for calculation methods and models are also higher.At present,there is only a few experimental reports on absorption spectra of SCI and substituted CIs(including methyl substituted Cl,i.e.,MCI and methyl vinyl ketone oxide,i.e.,MVCI).It is generally accepted that the two lowest excited states of Criegee intermediate are characterized by n??*and ???*transitions,respectively.Theoretically,it is inferred that there are two spin-allowed singlet dissociation channels in SCI:H2CO(X1A1)+O(1D)and H2CO(a 3A")+O(3P).In addition,there is few report about the photo-reaction mechanism of other complex CIs.As a large number of theoretical studies on the ground state reaction have shown significant substituents effects on the unimolecular thermal reactions of CIs,we suggest here that the substituent effects in the photoreactions of CIs can not be ignored.Basing on the above research,we are mainly committed to solving the following two scientific problems:one is to search all possible photoreaction channels of Criegee intermediates;the other is to explore the substituent effects and their action rules in Criegee intermediates in the photoreactions.By combining the electronic structure calculations at MS-CASPT2 level and the trajectories surface hopping(TSH)nonadiabatic dynamics simulations at CASSCF level,we carried a systematic and comprehensive study of the photo-induced unimolecular reactions of CIs.By exploring the influence of saturated methyl and unsaturated vinyl substitution on the photodissociation mechanism of CI,we reveal the role of substituent effects in the photo-chemistry of CI.The main results are as follows:(1)A dark-state-involved A1A?X1 photoisomerization channel of SCI that leads to a cyclic dioxirane is revealed for the first time.After being excited,decay to A1 A state from B1A state through internal conversion,or initially from A1A state,the nonadiabatic transition occurs at A/X-MECI(minimal-energy crossing)by rotating the C-O bond,and then close the C-O-O ring on the ground state.The C-O-O ring-closure to form dioxirane is the minimum energy reaction path of SCI from S1 state,which is spontaneous without potential barrier.Another possible photoisomerization reaction through the A/XMECI is antilsyn isomerization refers to C-O bond.Dynamic simulation shows that the yields of the two reactions are significantly higher than that of in-plane O-O dissociation,which fully proves the importance role of out-of-plane bond rotation in the photo decomposition of CI systems.(2)For the single-H substituted Criegee intermediates,different types of substituents regulate the photoisomerization mechanism of CI through hydrogen bonding,ring tension and ?-conjugation.The S1-state reaction of anti configurations of CIs substituted by saturated groups are dominated by the photoinduced ring-closure;while the syn configurations with C-H…O intromolecular hydrogen bond that prevent bond rotation show lower reactivity,and more likely to have an O-O bond dissociation.For CIs substituted by unsaturated groups,the O-O bond is greatly strengthened by the ?conjugation and therefore O-O dissociation is not supported,thus photoinduced ringclosure is the dominant reaction path in each configuration,but the hydrogen bond in syn configuration reduces the reaction activity significiently.(3)In CIs with both saturated and unsaturated substituents,the photodissociation of different configurations on the S1 state starts from the C-O-O ring-closure,the main difference is that the nonadiabatic transition of syn(C-O)configuration with hydrogen bond show relatively low rate.In addition,unlike single-H-substituted CIs,the two heavy groups make the C atom has obvious pyramidalization,which further leads to the"splitting" of the MECI into two different ones,and participate in the nonadiabatic processes of anti(C-O)and syn(C-O)configurations,respectively.In summary,the photoreaction mechanism of Criegee intermediates was predicted theoretically,and the effects of different substituents on the photochemistry were studied and summarized.The study here not only enriches the chemical reaction channels of Criegee intermediates,but also provides a theoretical reference for the follow-up studies of atmospheric chemistry of Criegee intermediates.