Theoretical Calculation Of The Impact Of Water Solvent Molecules Uracil Electronic Spectrum

Genomic information was encoded and transferred by DNA and RNA nucleic acids base moelcules. The property and dynamics of these base molecular excited state play a critical role in necleic acid photodamage. The goal of the photophysical intvestigation is to understand how excited states produce photochemistry outcomes.Thus, the steady and transient absorption and fluorescence spectroscopy are main and key contents in the field. In addition, the advences in quantum chemistry and computer technology make the computational chemical method also become an important skill to study bases excited states. In this thesis,we investigate keto-uracil the lowest singlet excited state both in vaccum and condensed phase using quantum chemsitry tools. The details are showed by following charpters:Chapter one:Briefly intrduce that the background of excited electric structure for nucleic acid base moelcule and uracil base structure and fucntions.Chapter two:The calculation of excited state for uracil under vacuum. Firstly, CASSCF and SAC-CI are used to calculate the singlet excited state structure of uracil. We investigate the influence of active space on the excited state energies and transtion dipole moments. The results show that CASSCF has a high performce at low time consumption, The same time, caculating time is also long than others. While for SAC-CI the more active orbits are, the energies are more stable. Secondly, effects of different DFT functionals on molecular excited state are studied. We separately choose three hybrid density functionals B3LYP, CAM-B3LYP and LC-WPBE and three kinds of pure density functionals LC-BLYP, LSDA, PBEPBE to calculate uracil molecules of the lowest singlet excited state. The results show that the long-range correction CAM-B3LYP functional is the best functional to calculate the uracil excited state.Chapter three:The study of excited state for uracil under micro solvent. Based on uracil with four water supermolecular structure in Improta etal JACS, we respectively use TDDFT coupled with PBE1PBE and the LC-BLYP functionals performed in Gaussian09and the same functionals performed in Gamess program. In addition, the linear scaling two-body Fragmental Molecular orbital (FMO2-TDDFT) also are used to calculate the excited state of supermolecule. The results show that the same DFT functional different package GaussianO9and Gamess programs has difference excited energies for n-π*and π-π*transitions but the energies gap is quite similar. Compared the tradational full TDDFT with FMO2-TDDFT results, FM02can get similar results and achieve linear scaling calculations.Chapter four:The calculation of uracil molecular excited state with many water surrouding molecules. Molecular dynamics simulation is applied to modeling the uracil and water conformation behavior. The lowest singlet excited states for uracil in water molecules are were calculated by the traditional TDDFT LCBLYP functional and the linear scaling FMO2-TDDFT. The results show that FMO2-TDDFT will use less computational time and can get the same excited energies with tradational TDDFT.Chapter five:Using FMO2-TDDFT, we calculated the excitation energise of the different conformations with massive water solvent. Increased as the number of water molecules, solvent contribution to uracil absorption spectrum over a long distance is very small.Chapter six:Summerization.