硝基烃光异构化反应的密度泛函理论计算

采用DFT(B3LYP)计算方法, 在6-31G*水平上获得了反式-β-硝基苯乙烯、硝基乙烯和硝基甲烷基态异构化反应时的过渡态分子结构, 并计算了异构化能垒及激发态电子跃迁能. 结果显示, 反式-β-硝基苯乙烯和硝基苯与硝基甲烷相比具有较短的过渡态C—N键长, 较低的异构化能垒, 并且随着不饱和度的增加, 硝基苯和反式-β-硝基苯乙烯电子垂直跃迁能与基态异构化反应过渡态之间能量的差值ΔE迅速减小. 从能量的角度分析, 取代基的不饱和度越大, 越有利于激发态势能面与异构化反应势能面发生锥型或漏斗交叉, 因而越有利于光化学反应沿光异构化通道进行. 激发态分子的初始电子运动的定域或离域特征的差别可能是导致硝基苯等硝基芳烃与硝基甲烷等硝基烷烃光解通道不同的一个重要原因.

Density Functional Theory Investigation of the Photoisomerization Reaction of Nitroalkanes and Nirroaromatic Compounds

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.