α、β硝基胍的异构化反应机理及其相对稳定性的密度泛函研究

Density Functional Theory Study on the Reaction Mechanism and the Relatively Stability of α- and β-nitro-guanidine Isomers

用密度泛函(DFT)中的B3LYP方法在6-311++G**水平上优化了α和β型硝基胍的分子结构,找到反应的过渡态,通过振动分析对异构体及过渡态进行了确认,并通过内禀反应坐标(IRC)计算确定了所得过渡态的真实性.对单点用QCISD(T)/6-311++G**计算了能量,同时进行了零点能校正.研究结果表明该反应是一个放热反应,放出热量为35.68kJ/mol,反应的活化能为125.68kJ/mol;β型硝基胍由于分子中存在共轭效应使键长趋于平均,体系能量降低,比α型硝基胍分子更趋于稳定;该反应的正、逆反应的速率常数都较小,分别为1.6×10-9s-1和4.71×10-12s-1,在常温下较难发生异构化,正反应较逆反应易于发生.

B3LYP/6-311 ＋ ＋ G ＊ ＊ method was performed to optimize the geometric configurations of α- and β-nitro-guanidine. The isomers and the transition state were confirmed by vibrational analysis, and the transition state was also confirmed by IRC calculation. The single point energy has been calculated at the QCISD（T）/6-311 ＋ ＋ G＊ ＊ level, and zero point energy correction has been considered. The results show that this is an exothermie reaction, and the heat effect is 35.68 kJ/mol and the activation energy is 125.68 kJ/mol. The eonjugative effect in fl-nitro-guanidine averages the bond length and reduces the energy, so β-nitro-guanidine is mere stable than α- nitro-guanidine. The rate constants of the forward reaction and the reverse reaction are small, and they are 1.6×10^-9s^-1 and 4.71×10^-12s^-1, respectively. At room temperatures, it is more difficult for an isomerization and the forward reaction is easy to take place.