Effect Of Twisted Angle Of Nitro Group On Excited State Dynamics Of Nitro Naphthalenes

Nitro polycyclic aromatic hydrocarbons?Nitro-PAHs?are mutagenic and carcinogenic environmental pollutants.Comprehensive studies showed that photolysis is the major pathway of their degradation in the environment.The Photochemical reaction process can be described as followed:After being excited to a high excited Sn?n?2?state,it will decay to singlet S₁???*?state through a quickly internal conversion?IC?.Irradiation of S₁???*?leads to the formation of intermediates mainly including the lowest excited triplet T₁ state,nitrogen oxide?NO^??and aryloxy radicals?ArO^??.It is worth noting that the structure of polycyclic aromatic hydrocarbons and the position of have a significant effect on the photolysis yields.Some photochemical measurements and quantum-chemical calculations showed the photodissociation yields were largely controlled by the dihedral angle between nitro plane and aromatic ring plane.However,the detail information about the Microscopic dynamic mechanism was unclear which needs higher level of quantum chemical calculations.Therefore,in this paper,we study the relaxation paths,isomerization reactions and the most probable dissociation channels of 1NN,2NN and2M1NN by using CASSCF//CASPT2 methods combined with the resonance Raman spectrum analysis.The study results are summarized as following:?1?We obtained ultraviolet absorption spectrum,fourier transform raman spectrum?FT-Raman?,fourier transform IR spectrum?FT-IR?and resonance Raman spectrum of 1NN and2M1NN.Then various vibration modes were identified according to the theoretical vibration spectrum of ground state with density functional theory?DFT?calculations.Furthermore,the transition properties,oscillator strengths and vibration frequencies were gained through time-dependent density function Theory?TD-DFT?calculation so as to research three compounds'ultraviolet absorption spectrum.The electron transitions of the strongest absorption band?A-band?are all belong to S₀?S₁??_H?_L^*?transition.?2?The structures and energy information of ground states,excited states,intersections and transition states for the three NNs and their isomers?hereafetr marked ISO?were optimized with the methods of CASSCF,CASPT2,QSD and so on.Results show that three materials all have a similar decay process::S_n,FC???*??S_n,???*?T_n?…?T₂T₁?T_1,min,min and/or ArO^?.It shows that S_n,FC???*?state undergoes intersystem crossing to T_n state then internal converses to T₂T₁,finally decays to T_1,min,min or to ISO which can easily dissociate to ArO^?/NO^?with no energy gap.?3?weconfirmtheimportanceof NO₂andit's isomerin excitedstate relaxation mechanisms and dissociation dynamics of NNs according to the following three aspects:Firstly,the potential energy surface scanning along N-O bond was carried out to determine the feasibility of the dissociation of isomers.It shows that the dissociation of ISOs is energy barrierless in both S₁ state and T₁ state,however,the ground state on the contrary.It shows that ArO^?radicals were produced from S₁ and T₁ state rather than the ground state.Secondly,by studying the differences of the attenuation mechanism between three materials from CASSCF calculation results,we conclude that the structural differences of nitro group and the energy barriers between T₂T₁ and the transition state of T₁?Ts?T₁??control the photochemical yields of ArO^?and T₁ state.The dihedral angles between nitro and aromatic ring?_ONCCNCC of transition states are respectively?16/92?°,?21/93?°and?26/92?°,and?35/1?°,?46/83?°and?62/77?°for T₂T₁ states.The dihedral angles of T₂T₁ and Ts?T₁?of 1NN are similar,but the N-O bond lengths are quite different.However,both dihedral angle and N-O bond length are similar for 2M1NN.And it will overcome a tiny energy gap from T₂T₁ state to Ts?T₁?for 1NN and2M1NN but a energy gap nearly 15kcal/mol for 2NN.This can well explain the experimental results:Dissociation reaction is most significant for 2M1NN,1NN behind it,but hardly observed for 2NN.Thirdly,we find part of the experimental evidences which can support the main relaxation mechanism indicated by the calculated results from resonance Raman spectra.No observable anti-symmetric O-N-O stretch mode and C-NO₂ torsion mode are displayed in the resonance Raman spectra both of 1NN and 2M1NN in cyclohexane,acetonitrile and methanol,shows that in Frank-Condon area,the role of direct isomerization reaction from S₁???*?to dissociative state as main decay channel can be ruled out.