Study Of Short-time Dynamics Of α,β-Enones From The Lowest Light-absorption State S(ππ*)

Potential energy surface intersection especially the conical intersection is the most effective way that excited state molecules make a radiationless transition in two different potential energy surface.Ultrafast femtosecond time-resolved spectroscopy is the most effective method to explore the non-radiative transition process near the intersection of potential energy surface,and plays a leading role in the molecular photochemical dynamics.However,it is difficult for ultrafast real-time time-resolved spectroscopy to provide the structural dynamics process during the time period from Franck-Condon(FC)region to the potential energy surface intersection.therefore,ultrafast real-time time-resolved technology will face challenges when there are multiple potential energy surface intersection to cause radiationless transition process.Resonance Raman spectroscopy combined with complete-active-space self-consistent-field(CASSCF)theoretical calculation method for solving the problem of structural dynamics during the period is the most effective method,and it plays an important role in explainingstates non-radiative dynamics of the molecular excited states.In this paper,the method of resonance Raman spectroscopy and CAS SCF calculation method are used to study the short-time dynamics of α,β-enones.Obtain the following results:(1)The time dependent density functional theoretical calculation and vibration mode analysis are carried out,and the ultraviolet absorption spectra and resonance Raman spectra are identified.The CASSCF calculation is carried out to obtain the optimal geometry and energy of each electron excited state,and the optimal structure and energy of the intersections of related potential surface.A-band resonance Raman spectrum of 4-methyl-3-entene-2-one are composed of base frequency,double frequency,multiple combination frequency of v8,v9,v1o,v13,v15,v18,v19,v20,v25 vibration modes,the base frequency,double frequency,multiple combination frequency of the C=C stretching vibration mode v9 is major.A-band resonance Raman spectrum of 3-methyl-3-entene-2-one are composed of base frequency,double frequency,multiple combination frequency of v8,v9,v16,v18,v21,v23,v24;v25 vibration modes,the base frequency,double frequency,multiple combination frequency of the C=C and C=O stretching vibration mode v9,v8 is major.B-band resonance Raman spectrum of phthalide are composed of base frequency,double frequency,multiple combination frequency of v6,v7,V8,v10,v12,v14,v15,v18,v21,v22 and V23 vibration modes,the base frequency,double frequency,multiple combination frequency of the C = C stretching vibration mode v6,C4=C5 and C7=C8 stretching vibration mode v7,v8 is major.C5=C6 and C8=C9 stretching vibration mode v8 is major in the resonance Raman spectrum.(2)A-band resonance Raman spectrum of 4-methyl-3-entene-2-one and 3-methyl-3-entene-2-one in cyclohexane solvent are carried out,then A-band absorption cross section and the absolute Raman cross section of every vibration mode in resonance Raman spectrum.In this paper,the intensity analysis of resonance Raman spectrum based on the time-dependent wave packet theory and Brown oscillator model were carried out,we had obtained the displacement of dimensionless normal major modes.We found out the problem of transforming the dimensionless normal displacement into internal coordinates,and obtained the dynamic parameters of internal coordinates with time.4-methyl-3-pentene-2-one is excited by ultraviolet light about 10 femtosecond time according to the results,its main bond length bond angle changes are consistent with the calculation result of S2S0 and S2S1 intersections structure relative to the ground by the CASSCF,it shows the wave packet has arrived at S2S0 and S2S1 intersection around on 10 femtoseconds.The important role of S2S0 and S2S1 intersections at 4-methyl-3-pentene-2-one in the process of excited states nonradiative relaxation was found.For the first time,the C=O double bond have a very fast conjugate or torsional motion relative to C=C double bond,which means that C=C-C=O conjugate system is destroyed by torsion in less than 2 femtoseconds.On time scales,3-methyl-3-pentene-2-one structure dynamics are similar to 4-methyl-3-pentene-2-one,Their main bond length bond angle changes are consistent with the calculation result of S2S1 intersection structure relative to the ground by the CASSCF when they was activated by ultraviolet light in the cyclohexane solvent about 10 femtoseconds,which shows that the wave packet has arrived at S2S1 intersection around on 10 femtoseconds.The S2S0 channels are not present in 3-methyl-3-pentene-2-one excited relaxation path.(3)B-band resonance Raman spectrum of phthalide in acetonitrile solvent are carried out,then B-band absorption cross section and the absolute Raman cross section of every vibration mode in resonance Raman spectrum.The intensity analysis of resonance Raman spectrum based on the time-dependent wave packet theory and Brown oscillator model were carried out,we had obtained the displacement of dimensionless normal major modes and obtained the dynamic parameters of internal coordinates with time.They reveals relaxation mechanism of phthalide excited state,that the major decay channel from excited state return to ground state through S3(2)/S1 intersection.