Experimental And Theoretical Studies On The Excited State Dynamics Of Aromatic Aldehydes

Aromatic aldehydes are a major source of carbonaceous pollution in the atmosphere.Studies have shown that the natural degradation or elimination of aromatic aldehydes in the atmosphere is mainly achieved through photochemical reaction processes.First,sunlight can cause the decomposition of some aromatic aldehydes.Second,the active molecular cleavage produced by photochemical decomposition,like OH,NO₃,etc,can also cause chemical degradation or conversion of aromatic aldehydes.At present,the photochemical attenuation pathway and quantum yield of aromatic aldehydes have not been completely determined,and the microscopic dynamic mechanism is still unclear.This thesis studies excited-state short-time structure dynamics and decay mechanism of benzaldehyde?BA?,methacrolein?MA?and 3-furaldehyde?3FA?by employ resonance Raman experiment technique combined complete-active-space self-consistent-field method?CASSCF?.Results are as follows:?1?The ultraviolet absorption spectrum and resonance Raman spectrum of benzaldehyde were obtained,and the electronic transition and vibration mode assignment of UV and Raman spectra were respectively determined by density functional theory calculation and PED?Potential energy distribution?analysis.The strongest absorption band of benzaldehyde?C band?belongs to the?_H-1??_L*electronic transition,which produces the excited state of S₃?2A'?.The optimized structure and energy information of the benzaldehyde ground state,low energy excited state and potential energy surface intersection were obtained by CASSCF and CASPT2,Then the short-time structural dynamics of benzaldehyde is obtained by analysis the the variation of the internal coordinates at 10 femtoseconds,which is transformed by displacement of the dimensionless vibrational normal mode obtained by the Resonance Raman spectral intensity fitting parameters.The results show that there is a vital potential energy surface intersection S₃S₂,After the molecular excitation is released to the higher excited S₃ state,it is internally converted to S₂,and then reaches the S_1-MIN via the fast relaxation channel IC?S₂/S₁?,and then turns to the triplet state by the tri-state intersection S₁/T₁/T₂ structure,finally emits phosphorescence back to the ground state.?2?Short-time structural dynamics of methacrolein upon excitation to the light-absorbing S₂???*?state were explored.The absolute UV absorption cross section and Raman cross-sectional area of vibration mode of methacrolein?MA?in a cyclohexane solvent was measured.Time-dependent wave packet theory was used in the simulation of the absorption cross section and resonance Raman absolute cross section in methacrolein.We acquired dimensionless normal mode displacements of 6 representative resonance Raman peaks.Thedimensionless normal mode displacements were further converted to short-time?10fs?structuraldynamics in terms of the internal coordinatesin Franck-Condon region.The main structural kinetics information is compared with the structure of S_2-MIN and S₂S₁ obtained by CASSCF calculation method.The important role of S₂S₁ intersection in the non-radiative relaxation of methacrolein excited state is found.It is indicated that in the Franck-Condon area,the wave packet mainly goes to the intersection of S₂S_1-MIN;part of the wave packet goes to S_2-MIN.?3?The ultraviolet absorption spectrum,Fourier Raman spectrum,Fourier infrared spectrum and resonance Raman spectrum of 3 furaldehyde were acquisitedand theassignment of experiment spectra was analysis by calculated theoretical vibrational spectra using Time-dependent Density Function Thero?TD-DFT?calculation.We also get the two structure of ground state at B3LYP/cc-PVTZ level,the trans isomer was proved to be the most stable configuration,at the same time,The electron transitions of the strongest absorption band?A-band?are belong to S₀?S₂???^*?transition.In order to investigate the relationship between potential energy surface intersection and resonance Raman intensity mode,the structures and energy information of excited states intersections and transition states for 3FA were optimized with the quantum chemical calculation methods such as CASSCF and CASPT2.It is found that similar to benzaldehyde,there is a fast relaxation channel with the same characteristics:S_2,FC???*??IC?S₂S₁??S_1-MIN?S₁T₂?or S₁T₂T₁??T₂T₁?T_1-MIN?S₀...