Study Of Excited-state Potential Energy Surfaces Crossing Dynamics Of Enones

The photochemistry and photophysics of α,β-enones has been the subject of manyexperimental and theoretical studies over the past half century，especially the lowest triplet andsinglet excited states (T1and S1) of the simplest α,β-enones, acrolein. The main characteristics ofα,β-enones compounds is the conjugated structure of carbonyl group and the carbon doublebond，which is similar to that of nucleic acid bases. It will have more advantages to research theexcited state structural dynamics by useing more simple structure compounds of α,β-enones overnucleic acid bases.The n orbit on C=O and π orbit on C=O and C=C of α,β-enones can transition to the π*orbitforming n�'π*and π�'π*electronic transition. Therefore, the electronic excited states ofS1(1nπ*) and S2(1ππ*) is very important to the photochemical reaction process of this compounds.In order to find the changes of structural dynamic information and regulation factors in theprocess of excited state decay dynamic after molecular excited to S2(ππ*) of this structure indifferent compounds. We select3kinds of α,β-enones compounds with similar structure as ourresearch system in this paper. Time-dependent density functional (TD-DFT) calculation andResonance Raman spectroscopy are used to explore the excited state structural dynamicsinformation. Then use the CASSCF (Complete active space the self consistent field) calculationto research the geometric structure of excited states and potential energy surface crossing point.We had obtained the electronic absorption spectra and some different wavelength resonanceRaman spectra of3-Methyl-3-buten-2-one(3M3B2O),3-methyl-3-pentene-2-one(3M3P2O)and4-methyl-3-pentene-2-one(4M3P2O) in methanol, acetonitrile and cyclohexane. The C=Ostretching vibration will have small red shift when the solvent polarity is stronger. But thechange of excitation wavelength has no effect on the spectrum.The geometric structure, vibrational frequencies and the electronic transition of the Compoundwere calculated by usying TF-DFT. Thus the strongest absorption band was assigned as theπH-1�'πL*transitions. Meanwhile, some fundamentals vibrational modes of the resonanceRaman spectra were identified Combining calculation with the results of experiment, predominately in the C=C stretch mode, C=O stretch mode. This indicates that excited stated structural dynamics is mainly along the C=O stretch and C=C stretch coordinates. The intensity change of v(C=C), v(C=O) in3M3B2O and3M3P2O indicate that the propagating wave-packet evolves time-dependently and nonlinearly along the combined C=C and C=O reaction coordinate in the S2(1ππ*) potential energy surface. At the very beginning the wave-packet evolves along the major C=C stretch coordinate for some distance first, and then more and more along the C=O stretch coordinate in addition to the major C=C stretch coordinate. But in4M3P2O the propagating wave-packet is linearly along the combined C=C and C=O reaction coordinate. The relative intensity of v(C=C) and v(C=O) in A-band resonance Raman spectra of3different materials is change along with the difference of methyl position and number. The intensity of v(C=O) and nv(C=O) is weakened sharply as the molecule goes from3M3B2O to3M3P2O and to4M3P2O. In order to explain this phenomenon from the aspects of theory, The geometry structures of the excited states and the potential energy surface crossing point of3M3B2O,3M3P2O and4M3P2O has been studied using CASSCF computations at the CASSCF(6,5)/6-31g (d) leavel. The calculation results show that:1、The biggist change of geometry structures of the excited states and the potential energy surface crossing point is C=C and C=O bond length elongation, which is fits with the experimental results.2、The intensity of v(C=O) and nv(C=O) is weakened sharply as the molecule goes from3M3B2O to3M3P2O and to4M3P2O, it has nothing to do with the C=C and C=O bond length changes, but relate to dihedral angle of O=C-C=C-C(M) of the S1(nπ*)/S2(ππ*) geometry.3、The most advantageous radiationless decay routes from S2, FC to the ground state is through S1(nπ*)/S2(ππ*) and S1(nπ*)/S0.