硝基甲烷异构化反应势能面的ab initio研究

在B3LYP/6-311+ +G(2d,2p)水平上,优化得到硝基甲烷CH3NO2的10种异构体和23个异构化反应过渡态,并用G2MP2方法进行了单点能计算.根据计算得到的G2MP2相对能量,探讨了CH3NO2势能面上异构化反应的微观机理.研究表明,反应初始阶段的CH3NO2异构化过程具有较高的能垒,其中CH3NO2的两个主要异构化反应通道,即CH3NO2→CH3ONO和CH3NO2→CH2N(O)OH的活化能分别为270.3和267.8 kJ/mol,均高于CH3NO2的C-N键离解能.因而,从动力学角度考虑, CH3NO2的异构化反应较为不利.

Ab initio Study of the CH3NO2 Rearrangement Potential Energy Surface

The potential energy surface of nitromethane (CH3NO2), including 10 CH3NO2isomers and 23 inter-conversion transition states, is probed theoretically at G2MP2//B3LYP/6-311++G(2d,2p) level of theory. The geometries and relative energies for various stationary points are determined. Based on the calculated G2MP2 potential energy surface, the possible nitromethane isomerization mechanism is discussed. The results are shown that the energy order for these 10 CH3NO2isomers is IS6>IS5>IS3>IS8>IS2>CH3NO2>IS4>IS10>IS9>IS7. The energy of IS7b (HC(O)N(H)OH) is the lowest, 111.7 kJ/mol below the nitromethane (CH3NO2), while the energy of IS6C (HCN(OH)2) is the highest, 240.6 kJ/mol above the nitro methane (CH3NO2). It is shown that CH3NO2isomerizations at the initial reaction stages are of high activation barriers. Among them, the respective nitromethane→methyl nitrite and nitro methane→aci-nitromethane barriers are 270.0 and 267.5 kJ/mol, higher than the C-N bond dissociation energy for CH3NO2. Our results suggest that nitromethane isomerization pathways are kinetically disfavored.