关键词: 光异构化, 电子跃迁能, 密度泛函理论, 硝基烃

Abstract: The geometry structures and the energy barriers for the ground-state isomerization reactions of nitromethane, nitroethylene, nitrobenzene, and trans-β-nitrostyrene were computed using B3LYP/6-31G* level of theory. Their electronic transition energies were obtained using B3LYP-TD/6-31G* calculations. The results indicated that the C—N bond lengths of trans-β-nitrostyrene and nitrobenzen were significantly shorter than that of nitromethane, meanwhile the isomerization energy barriers of trans-β-nitrostyrene and nitrobenzen were somewhat lower than that of nitromethane. The energy difference (ΔE) between the vertical electronic transition energy and the transition state of the ground-state isomerization decreased dramatically with the increase of the molecular unsaturation as the molecule went from trans-β-nitrostyrene to nitrobenzene and to nitromethane. This suggests that as the unsaturation degree of the substituent R increases for R—NO2, the curve crossing between the excited state and the ground isomerization potential energy surface increases and this greatly favors the isomerization exit channel. The calculated first transition-allowed absorption band (also called A-band absorption) was assigned to a π→π* transition for these molecules. While the A-band electronic transition of nitromethane is mainly localized on the NO2 group, those of trans-β-nitrostyrene, nitrobenzene and nitroethylene are largely delocalized over the molecules, which causes the intramolecular charge-transfer processes to take place between the NO2 group and the C6H5—C=C group, the C6H5 group as well as the C=C group. Thus the localization or delocalization of the A-band electronic transition of nitroalkanes or nitroaromatics plays an important role in manipulating its photodissociation channel or photoisomerization channel.

Key words: Photoisomerization, Electronic transition energies, Density functional theory, Nitro-compounds