The Theoretically Studies On The Isomerization Mechanism Of β Substituted Styrene

Recently, the mechanism study of the isomerization of the conjugated alkenes becomes one of the most active fields. The paper applies the quantum chemistry methods and systematically studied the molecular mechanisms of E/Z-isomerization of styrene, 1-methyl-2-phenylethene, 1-alkoxy-2-phenylethene and 1-[1,3]dioxane-2-phenylethene. All these compounds are compounds ofβ-substitutional styrene.Firstly, the molecular mechanisms of the thermal E/Z-isomerization of styrene, 1-methyl-2-phenylethene, 1-alkoxy-2-phenylethene and 1-[1,3]dioxane-2-phenylethene are investigated theoretically. Using the ab initio and density functional theory (DFT), the molecular mechanisms of E/Z-isomerization of these compounds are investigated by calculating the stationary and transition structures, relative energies and charge distributions. It is found that the reaction process of the isomerization of the compound included in the compounds ofβ-substitutional styrene passes by a transition structure in which the double bond is elongated to single bond and the reaction activation energy is very high. Theseβ-substituents have significant effect on the activation energy, and the activation energy of the isomerization of 1-methyl-2-phenylethene is 48.2 kcal mol-1, which is the lowest isomerization energy in these four compounds.Secondly, the absorption spectrum, emission spectrum and frontier molecular orbital of E/Z-isomers of styrene, 1-methyl-2-phenylethene, 1-alkoxy-2-phenylethene and 1-[1,3]dioxane-2-phenylethene are investigated by using TD-DFT, CIS and MC-SCF methods, respectively. The results of maximum absorption wavelength calculated at TD-DFT/B3LYP methods are identical with the experimental results. However the electronic transition energy is overestimated at the CIS and MC-SCF methods. The variation tendency of absorption wavelength for compounds ofβ-substitutional styrene is identical at three theoretical methods. The variation tendency of absorption wavelength is also consistent with the variation tendency of the energy gap. From the analyse of frontier molecular orbital, it is found that the electron excitation benefits to the E/Z-isomerization of theseβ-substitutional styrene. The emission spectrums of these compounds are all red shift than the absorption spectrum, which is agreement with the experimental results. The study of optical and electronic properties is the foundation for the investigation of the molecular mechanisms of E/Z-photoisomerization.At last, the mechanisms for the E/Z-photoisomerization of styrene, 1-methyl-2-phenylethene, 1-alkoxy-2-phenylethene and 1-[1,3]dioxane-2-phenylethene are investigated at the MC-SCF theory method. It is found that two local minima also exist on the first excited state(S1) potential energy surface and the E-isomer is slightly more stable than Z- isomer on the S1 surface. In consistent with the ground state, there is an energy barrier on the S1 surface. The energy barriers of the E/Z-photoisomerization of 1-methyl-2-phenylethene, 1-alkoxy-2-phenylethene and 1-[1,3]dioxane-2-phenylethene are increased with the augment of the substituents.The molecular mechanisms of E/Z-isomerization of styrene, 1-methyl-2-phenylethene, 1-alkoxy-2-phenylethene and 1-[1,3]dioxane-2-phenylethene have been analyzed and investigated theoretically. The results indicate that the MC-SCF theory can provide reasonable geometrical structures and relative energies, which can be applied to the theorical study of other reactions mechanism.