Abstract
The detailed mechanism of the NO(2)+CH(4) reaction has been computationally investigated at the M06-2X/MG3S, B3LYP/6-311G(2d,d,p), and MP2/6-311+G(2df,p) levels. The direct dynamics calculations were preformed using canonical transition state theory with tunneling correction and scaled generalized normal-mode frequencies including anharmonic torsion. The calculated results indicate that the NO(2)+CH(4) reaction proceeds by three distinct channels simultaneously, leading to the formation of trans-HONO (1a), cis-HONO (1b), and HNO(2) (1c), and each channel involves the formation of intermediate having lower energy than the final product. The anti-Hammond behavior observed in channel 1a is well analyzed. Proper treatment of anharmonic torsions about the C···H···O (or N) axis in the transition structures greatly improves the accuracy of kinetics predictions. The activation energy for each channel increases substantially with temperature, but is not strictly a linear function of temperature. Therefore, the thermal rate constants are fitted to the four-parameter expression recommended for this case over the wide temperature range 400-4000 K.
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