The reaction of CH3 radical with molecular O2 has been investigated by ab initio molecular orbital theory and variational transition state theory calculations. The detailed potential energy surfaces, including the crossing seams between the PES, located by means of the intrinsic reaction coordinate approach are presented. The rate constants for the association and product formation channels have been calculated and compared with the experimental data. Under the atmospheric pressure condition, the association reaction (a) producing CH3O2 dominates reaction below 1500 K. The branching probabilities for channels (b) and (c) producing CH2O+OH and CH3O+O, respectively, have been calculated and compared; channel (b) is predicted to be dominant below 2000 K with the rate constant kb=1.14×10−22T2.86exp(−5120/T) cm3 molecule−1 s−1. Over 2000 K, channel (c) becomes competitive; its rate constant could be represented by kc=1.01×10−16T1.54 exp(−13 280/T) cm3 molecule−1 s−1 in the temperature range of 1000–3000 K. In addition, the most exothermic products, CHO+H2O, were found to be kinetically inaccessible because of the large barrier, 47.4 kcal/mol above the reactants.
The reaction of HCO with O2 has been studied by ab initio molecular orbital and statistical theory calculations. Both the direct abstraction and the association–elimination processes have been considered. The direct abstraction of H by O2 producing the HO2+CO products was found to be unimportant below 2000 K. The association reaction occurs by the attack of O2 at the C atom to form a vibrationally excited complex, HC(O)OO°, which can undergo two reactions. The first possibility is H migration via TS2 forming HOOCO, which rapidly dissociates into either OH+CO2 via TS4 or HO2+CO via TS3; the latter is energetically less favorable. The second possibility is the direct production of HO2+CO from HC(O)OO° via TS5 in a concerted manner. The barrier of TS5 at the G2M level of theory is 23.5 kcal/mol relative to HC(O)OO; this is the major channel for the reaction. Variational transition state theory and Rice–Ramsperger–Kassel–Marcus calculations have been carried out for the direct abstraction and the indirect metathetical mechanisms, respectively. The calculated total rate constant at 1.5 Torr exhibits a small positive activation energy and its absolute values agree closely with experimental data.
Potential energy surface of the reaction of NH2 with NO2 has been studied at the QCISD(T)/6-311G(d,p)//MP2/6-311G(d,p)+ZPC[MP2/6-311G(d,p)] and GAUSSIAN−2 (G2) levels of calculation. The reaction is shown to give three different groups of products. H2NO+NO can be produced by two different channels: (i) the barrierless association of the reactants to form H2NNO2 1, followed by the nitro–nitrite rearrangement into H2NONO 3 and the ON bond scission and (ii) the association of H2N with ONO directly forming 3 without barrier, followed by the dissociation 3. The barrier for the nitro–nitrite rearrangement at the transition state (TS) 2, 31.2 kcal/mol with respect to 1, is 20.8 kcal/mol lower than the reactants at the best G2 level. The TS 2 is found to lie significantly lower and to have much tighter structure than those previously reported. The thermodynamically most stable N2O+H2O products can be formed from 1 by the complex mechanism (iii), involving 1,3-hydrogen shift from nitrogen to oxygen, rotation of the OH bond, H shift from one oxygen to another and migration of the second H atom from N to O leading to elimination of H2O. The rate-determining step is the 1,3-H shift at TS 4 which is 12.5 kcal/mol lower than NH2+NO2, but 8.3 kcal/mol higher than the barrier for the nitro–nitrite isomerization at TS 2 at the G2 level. N2+H2O2 cannot be formed in the reaction, but several channels are shown to produce N2+2OH. All of them have as the rate-determining step the second 1,3-hydrogen shift from nitrogen to oxygen at TS 11 or 16, lying by 6.9 kcal/mol higher than NH2+NO2, and are not expected to compete with the reaction mechanisms producing H2NO+NO and N2O+H2O.
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