Theoretical Calculation Of The Excited State Potential Energy Surfaces And Dynamics Of ?-? Enones And Pyrimidine Bases

The combination of femtosecond measurement technology and quantum chemical calculation has become a powerful method for comprehensive mapping of excited state dynamics.However,due to the limitations of the uncertainty law,the most advanced femtosecond time-resolving technology is difficult to directly measure the evolution of molecular structure over time in 50 femtoseconds,which makes the pyrimine base excited state relaxation mechanism revealed by femtosecond time-resolved electron spectroscopy technology still controversial.Resonance Raman spectroscopy intensity analysis provides a unique means of collecting this information and can provide new and critical data for resolving this controversy.Therefore,the density functional theory?DFT?and the complete active space self-consitent field?CASSCF?theory are used to study the excited state structure,potential energy surface and relaxation dynamics of?,?-enones and pyrimidine.Based on the experimental facts of resonance Raman spectroscopy,the theoretical calculations of the excitation dynamics of?,?-enones and pyrimidine were carried out.The relationship between resonance raman spectral intensity model,molecular excited state structure,conical intersection and initial polarization was clarified in?,?-enones,and the essence of the initial structural dynamics of uracil and thymine in aqueous solution was revealed.Results are as follows:?1?The structure of ground state,excited state and conical intersection of methacrolein?MA?,3-methyl-3-butene-2-one?3M3B2O?,3-methyl-3-pentene-2-one?3M3P2O?and4-methyl-3-pentene-2-one?4M3P2O?were obtained by CASSCF theoretical calculation.The energy was corrected by using MS-CASPT2/aug-cc-PVTZ calculation level.In addition,time-dependent density functional theory?TD-DFT?was used to calculate the electronic transition energy,transition orbit and oscillator strength.?2?It was found that the singlet state transitions through the S₂???*?/S₁?n?*?intersection in the?,?-enones.In the triplet state,the calculated spin-orbit coupling value of S₂/T₃ is less than20 cm^-1,which does not conform to the EL-sayed rule,so the efficiency of crossing to T₃between S₂ States is very low.Since the structural energies of S₁/T₂ and S₁/T₁ are similar,we believe that there exists a three-state intersection ISC[S₁/T₁/T₂].As the intersection point of S₁,T₁ and T₂ states,it plays a crucial role in decay.Based on the results of the study,there were three decay paths for?,?-enones:?path I?S_2,FC?S_2,min?S₂/S₁?S_1,min?S₁/S₀?S₀;?path II?S_2,FC?S_2,min?S₂/S₁?S_1,min?ISC[S₁/T₁/T₂]?T_2,min?T₂/T₁?T_1,min?T₁/S₀?S₀;?path III?:S_2,FC?S_2,min?S₂/S₁?S_1,min?ISC[S₁/T₁/T₂]?T_1,min?T₁/S₀?S₀.The ground state and excitation potential energy curves of cis-trans isomers were obtained by linear internal difference method,and the maximum ground state energy barrier was less than10 kcal/mol,which indicated that molecule could be isomerized in ground state.In the excited state,it is difficult for the molecules to isomerize because of the high energy barrier or the large difference in structure.?3?The most remarkable characteristic of resonance Raman spectra of different?,?-enones is the change of Raman intensity of C=O stretching mode.With the increase of methyl substitution on C_?and C?,the molecular torsion speed will increase,thus accelerating the molecular de-conjugation.As a result,a pseudo-planar polarization⁺C?-C_?=C-O^-dominates the initial structure dynamics of MA,so that largest intensity borrowing occurs to the C=O stretch mode of MA.In 4M3P2O,the methyl group twists rapidly?less than 1 fs?,causing serious distortion or even vertical polarization of C?-C_?=C-O group,resulting in a weak C=O peak in the spectrum,suggestting that 4M3P2O mainly dacy through S₂???*?/S₀.The intensity of the C=O peak can remack initial polarization of the molecule,and the initial polarization of the molecule shows the choice of the path between the FC region to the intersection.?4?The excited state structure of thymine and uracil in aqueous solution was obtained,and it was found that S₂???*?/S₁?n?*?is a pseudo-planar structure,and S₂???*?/S₀.is a vertically polarized structure.Combining?,?-enones research conclusions,If the excited state wave packet in the thymine aqueous solution moves mainly along S₂???*?/S₀,only a weak C=O peak can be detected in the resonance Raman spectrum,which is consistent with the experiment.Therefore,the time for thymine deconjugation is similar to that of 4M3P2O.In uracil,the wave packet first reaches the vicinity of the S_2,min or S₂???*?/S₁?n?*?,and then through S₂???*?/S₁?n?*?or S₂???*?/S₀ to the ground state.