Ab initio study of rearrangements on the nitromethane potential energy surface

A theoretical study has been performed for the thermal rearrangements connecting nitromethane, methyl nitrite, nitrosomethanol, and aci-nitromethane by using the 6-31G* basis set to optimize geometries and introducing correlation at the MP2 level. The lowest calculated rearrangement pathways from nitromethane are to methyl nitrite (..delta..H/sup double dagger/ = 73.5 kcal/mol) and aci-nitromethane (..delta..H/sup double dagger/ = 75.0 kcal/mol). Nitrosomethanol, although predicted to be only 1.6 kcal/mol less stable, is separated by two high barriers from nitromethane. An elimination transition structure connecting methyl nitrite and formaldehyde plus nitroxyl is found to have an enthalpic barrier of 44.1 kcal/mol. Fragmentation reactions have enthalpies of reaction less than calculated barrier heights, suggesting that concerted rearrangements on the CH/sub 3/NO/sub 2/ surface in general will not be observed.