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.