Nitromethane-methyl nitrite rearrangement:: A persistent discrepancy between theory and experiment

We reexamined the mechanism of the unimolecular rearrangement connecting both nitromethane and methyl nitrite isomers. The CH3NO2 potential energy surface was constructed using different molecular orbital [CCSD(T) and CASSCF] and density functional theory (B3LYP) methods including a few lower lying isomeric intermediates. A particular attention has been paid to the two following questions left open by earlier experimental and theoretical studies: (a) does the interconversion between nitromethane and methyl nitrite by a 1,2-CH3 migration occur via a loose or tight transition structure (TS)? and (b) is the energy barrier associated with methyl migration actually smaller or larger than the C-N bond dissociation energy? The C-N bond dissociation energy was evaluated with BDE(CH3-NO2) = 60+/-2 kcal/mol in line with available results. In contrast to earlier studies (McKee, M. L. J. Phys. Chem. 1989, 93, 7365, and Saxon, R. P.; Yoshimine, M. Can. J. Chem. 1992, 70, 572) but partly in agreement with a recent G2MP2 study (Hu, W. F.; He, T. J.; Chen, D. M.; Liu, F. C. J. Phys. Chem. A 2002, 106, 7294), our multiconfigurational CASSCF computations demonstrated that the methyl migration involves a tight TS whose electronic wave function is dominated by the Hartree-Fock configuration. Calculations are thus internally consistent indicating that the energy of the TS for 1,2-CH3 shift is at least 6 kcal/mol above the CH3 + NO2 asymptote. Thus, a discrepancy with a previous evaluation of experimental findings (Wodtke, A. M.; Hintsa, E. J.; Lee, Y. T. J. Phys. Chem. 1986, 90, 3549), which placed the CH3 + NO2 limit by 5 kcal/mol above the rearrangement TS, appears to persist.