Study Of The Photoinduced Dynamics Of Pyrimidine And Pyrazine | Posted on:2013-11-22 | Degree:Master | Type:Thesis | Country:China | Candidate:J L Guo | Full Text:PDF | GTID:2231330371486172 | Subject:Applied Chemistry | Abstract/Summary: | PDF Full Text Request | Pyrimidine and pyrazine (isomers), the female nuclear of many important heterocycliccompounds, have wide application, of which pyrimidine ring is most important, such as thebases, and three kinds of nucleic acid are pyrimidine derivatives. The electronic excited statesof nucleic acid bases play a central role in the UV light damage of nucleic acid. So thephotochemical and photophysical properties of pyrimidine and pyrazine, especially therelaxation dynamics process of excited states is the focus of attention. The lifetimes of thesecond electric dipole-allowed state (S2) of pyrazine have been reported to be very short (about22fs or so) in the literature, and fast decay to the ground state via conversion has beenproposed. However, conical intersection of electronic potential energy surfaces(PES) plays animportant role in the process of radiationless decay, ultrafast internal conversion and so on.Therefore, the information on conical intersection which gained from the study of theexcited-state PES can help us to reveal the excited-state dynamics of the molecular.As a very efficient and sophisticated means, Resonance Raman spectral technique has aunique advantage on the research of the conical intersection (IC), vibronic coupling in theFranck-Condon region, and other aspects. In this paper, the short-time dynamics ofphoto-induced Charge-Transfer of pyrimidine (PyM) and pyrazine (PyZ) in different kinds ofsolvents have been examined respectively by the Resonance Raman spectra. We also discussedthe influencal factors of the photochemical reaction in micro dynamics and the opticaltransformation mechanism, combined with the density functional theory (DFT) and completelyactivation of the space the self-consistent method (CASSCF). Main contributions and results ofthe present work are summarized as follows:(1) We have obtained the absorption spectrum of pyrimidine in four different kinds ofsolvents (cyclohexane, acetonitrile, water and methanol),all of which mainly have two allowedelectronic transition band respectively, and little influence of solvent polarity on the absorptionstrength and the displacement spectral peak of ultraviolet spectrum has been observed.Resonance Raman spectrums of pyrimidine were obtained in different solvents(cyclohexane andwater) with the208.8,228.4,235.9,266.0,273.9and282.4nm excitation wavelength in resonance with the CT-band absorption spectrum. Most of the B-band Resonance Raman spectraof pyrimidine can be assigned to the fundamentals, overtones and combination bands of aboutthree Franck-Condon active modes: the (C2,C5) ring deformation vibration mode14, the(N1,N4,C6) ring deformation vibration mode16and the (C3,C6) ring deformation vibration mode21, accompanied by a moderate degree of the motion along by5,6,9,12and22at the sametime. This indicates that the short-time structural dynamics of pyrimidine has obviousmultidimensionality. The similarities and differences of A-band, B-band and C-band excitedstate structural dynamics of pyrimidine were studied. The effect of solvent for its ResonanceRaman spectra is weak.(2) The electronic excitations and the Franck-Condon regional excited state structuraldynamics of pyrazine were studied by using the Resonance Raman spectroscopy and the densityfunctional theory(DFT) calculations. The impact of polarity solvent to ultraviolet spectrum is notbig. We have chosen to model the relative intensities of the245.9,252.7and266nm ResonanceRaman spectra and absorption spectra of pyrazine using the time-dependent wave-packetcalculations. The results indicate that there is good agreement between the calculated andexperimental absorption spectra while simultaneously providing a good fit to the absolute Ramanintensities of the Resonance Raman spectra. It also reveals that the short-time structuraldynamics of pyrazine in Franck-Condon region have multidimensional characters with motionpredominantly along the ring breath vibration mode4and the ring deformation vibration mode5, followed by C-H bending+C=C stretch vibration2and C-H bending vibration3. Wereport the vibronic-couplings and the excited state reaction dynamics of pyrazine in the S2(1B2u,1*) state to provide detail information about the curve-crossings between the bright S2stateand the low-lying dark states. More than three states are discerned to exist somewhere awayfrom the Franck-Condon region and couple with the bright S2(1B2u,1*) state throughHerzberg-Teller vibronic coupling mechanism. The wavelength-dependent and/or solvent-dependent of the curve-crossings are examined. The similarities and differences of A-band,B-band and C-band excited state structural dynamics of pyrazine were studied.(3) Two conical intersection points CI[S2(B2u)/S1(B3u)] and CI[S1/S0] and one transitionstate TS of the ground state isomerization reaction between pyrazine and pyrimidine arepredicted at the CASSCF(6,6)/6-31G(d) and/or B3LYP/6-31G(d) level of theories. At the same time, we gained the electronic excitation energies for the excited states S1and S2of pyrazine, theconical intersections of pyrazine, the transition barrier TS, their geometric structures and thecorresponding parameter values. Summarize the photophysical processes and photoisomerizationreaction between pyrazine and pyrimidine: first, pyrazine molecular was excited from its groundstate S0to the excited state S2, after arrive to the conical intersection CI(S2/S1), then through theconical intersection CI(S1/S0), and eventually some of the molecular get to the ground statepotential surfaces of pyrimidine molecules along the transition state TS; while the other part ofpyrazine molecular directly relax from the conical intersection CI(S1/S0) to the ground state S0ofpyrazine along its ground state potential surfaces.Finally, we have contrasted the excited state short-time structural dynamics of pyrimidinewith that of pyrazine, and summarize the differences and similarities.It found that they havesimilar vibration modes, but the Franck-Condon regional excited states structural dynamics havea few differences. Therefor, the shuttling motion of atom N between pyrimidine and pyrazineaffects their photochemical reaction dynamics of behavior. | Keywords/Search Tags: | Resonance Raman spectra, Excited state structural dynamics, Electronicvibronic-coupling, Time-dependent theory, photoisomerization | PDF Full Text Request | Related items |
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