Nitromethane−Methyl Nitrite Rearrangement:  A Persistent Discrepancy between Theory and Experiment

Abstract

We reexamined the mechanism of the unimolecular rearrangement connecting both nitromethane and methyl nitrite isomers. The CH 3 NO 2 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-CH 3 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(CH 3 -NO 2 ) ) 60 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-CH 3 shift is at least 6 kcal/mol above the CH 3 + NO 2 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 CH 3 + NO 2 limit by 5 kcal/mol above the rearrangement TS, appears to persist.

Isomers and isomerization reactions of four nitro derivatives of methane

“…Second, we gave a comparison of the most concerned relative energies (RE, relative to NM) of interested species. The data in Table 1 show us that our RE results are comparable with those derived from other high-level calculations: (1) for CH 3 1 NO 2 , our B3LYP/6-31111G(3df,2p) result, 59 kcal/mol, is in excellent agreement of the UCCSD(T)/cc-pVQZ (or CBS)//UB3LYP/6-3111G(3df,2p) and MRCISD(10,10)/(aug)-cc-pVTZ//CASSCF(10,10)/cc-pVDZ results of Zhu and Lin 3 and Nguen et al, 10 58.6 (59.8) and 59.2 kcal/mol, respectively, not more than 1 kcal/mol; 64 and 63 kcal/ mol of RE resulted from our and Nguen et al's 10 CCSD(T)/ccpVTZ calculation agrees well with each other; (2) for trans-CH 3 ONO, it seems our CCSD(T) result is better than our B3LYP result for it is closer to high-level MO calculation results; (3) for trans-CH 2 NOOH, our results, 19 and 22 kcal/mol, are very close to G2MP2 result, 21.4 kcal/mol 15 ; (4) and (5) for two TSs of CH 3 NO 2 to trans-CH 3 ONO and of CH 3 NO 2 to trans-CH 2 NOOH, our results are in accord with other results; (6) considering the RE of all species shown in Table 1, the B3LYP and all high-level MO methods give a same increasing order of RE: CH 3 NO 2 , CH 3 ONO, CH 2 NOOH, CH 3 1 NO 2 , TS (CH 3 NO 2 -trans-CH 2 NOOH), TS(CH 3 NO 2 -trans-CH 3 ONO). It shows that both geometry structures and energies predicted by B3LYP/6-31111G(3df,2p) are comparable with those predicted by the high-level methods.…”

FT-IR spectroscopic investigation of the surface reaction of CH4 with NO x species adsorbed on Pd/WO3–ZrO2 catalyst

“…The theoretical calculations on the homogeneous isomerization and dissociation of nitromethane show that both reaction pathways passing through an initial nitromethane -methyl nitrite and nitromethane -aci-nitromethane rearrangements have very close activation barriers differing by 0.6 [63] or 3 kcal/mol [64]. This leads to the conclusion that both channels are possible and competitive [64]. The presence of a catalyst can make one reaction pathway more favorable than the other.…”

Frozen natural orbital coupled-cluster theory: Forces and application to decomposition of nitroethane

“…Nitroethane is a prototype for the decomposition of the nitroalkane class of high-energy materials. 35,36 When compared to the better studied nitromethane, [37][38][39] the decomposition of nitroethane introduces one important additional pathway: The elimination of HONO. 40,41 This pathway is apparently the kinetically favored one for thermal decomposition of nitroethane, and is important in the decomposition of more complicated materials, such as 1,3,5-trinitrohexahydro-1,3,5-triazine ͑RDX͒.…”