Computational Study of the Reaction of Dimethyl Ether with Nitric Oxide. Mechanism and Kinetic Modeling

Abstract

To probe into the autoignition effect of nitric oxide (NO) on the combustion of dimethyl ether (DME), a detailed mechanism study and kinetic modeling for the reaction of DME with NO, which was considered to be very sensitive to the ignition delay time of DME, have been conducted using computational chemical methods. The CCSD­(T)/6-311+G­(2df,2p)//B2PLYP/TZVP compound method was employed to obtain the potential energy surface along the reaction coordinate, with the geometries, gradients, and force constants of nonstationary points calculated at the B2PLYP/TZVP theoretical level. The temperature-dependent rate coefficients from 200 to 3000 K were calculated using multistructural canonical variational transition-state theory (MS-CVT) with torsional motions and multidimensional tunneling effects included. The CCSD­(T) calculations with both 6-311+G­(2df,2pd) and cc-pVTZ basis sets give a zero-point inclusive barrier of 197–201 kJ mol–1 using the BMK/MG3S, B2PLYP/TZVP, and mPW2PLYP/TZVP based geometries. A van der Waals postreaction complex appears on the products HNO + CH3OCH2 side of the transition state. Two highly coupled torsions lead to four conformers for the transition state, and contributions from multiple structures and torsional anharmonicities substantially affect the rate coefficient evaluations. Variational effects can be argued to play an important role, especially at high temperatures, and tunneling probabilities increase with decreasing temperature. Because of large temperature-dependent feature of activation energy, the four-parameter formula 1.912 × 1011(T/300)3.191 exp­[−178.417­(T – 2.997)/(T 2 + 2.9972)] cm3 mol–1 s–1 is recommended for the MS-CVT calculated rate coefficients including small-curvature tunneling. The kinetic model is shown to give a satisfactory interpretation of the inhibited and accelerated effect of NO on the oxidation of DME.