Understanding and modeling the hydrogen-abstraction from dimethyl ether by the methyl radical with torsional anharmonicity

Abstract

Hybrid density functionals M06-2X and BMK in combination with the MG3S basis set, along with double-hybrid density functional B2PLYP with the TZVP basis set, were employed to better characterize the hydrogen atom transferring from dimethyl ether (DME) to the methyl radical. The distinct orientations of hydrogen atoms (in-plane and out-of-plane ones) in the DME molecule were taken into account. Density functional energetics was validated by comparison with the high-level CBS-QB3, G4, and G4MP2 calculations. Kinetic calculations were performed by means of the conventional transition-state theory (TST), canonical variational TST (CVT), and improved CVT (ICVT) over a wide temperature range 200–2600K to clarify the available experimental measurements, and tunneling effect and anharmonic torsion were also included. Two transition structures are located with hydrogen-abstraction occurring at the in-plane and out-of-plane hydrogen positions, which are in fact related via the relative motion of the CH3⋯H⋯CH2 and OCH3 moieties about the CO bond. Based on a single reaction pathway, the M06-2X/MG3SICVT rate constants with multidimensional small-curvature tunneling correction and proper treatment of anharmonic torsions compare well with most of the available experimental data. Variational effects on the computed rate constants are found to be negligibly small. Activation energies for the CH3+DME reaction increase substantially with temperature, and exhibit a nonlinear dependence on the temperature. Therefore, rate constants are fitted to the four-parameter expression k(T)=8.59×10-16(T/300)5.11exp[-19.65(T+140.90)/R(T2+140.902)] cm3molecule−1s−1 over the broad temperature range.