Hydrogen transfer between dimethyl ether and the methoxy radical: Understanding and kinetic modeling with anharmonic torsions

Abstract

Hybrid density functionals M06-2X and BMK with the MG3S basis set, as well as double-hybrid density functional B2PLYP with the TZVP basis set, were employed to study the reaction fundamental and kinetics of hydrogen-abstraction from dimethyl ether (DME) by the methoxy radical. The density functional energetics was validated against the calculations of the CBS-QB3, G4, and G4MP2 composite methods. Thermal rate constants were evaluated using the multi-structural canonical variational transition-state theory (MS-CVT) with multidimensional tunneling effect over the temperature range 200–2800K. It is found that the CH3O+DME reaction proceeds via a pre-reaction CH3O–DME adducts with subsequent intramolecular hydrogen transfer to form post-reaction CH3OH–CH3OCH2 complex and dissociation to produce the separate CH3OH and CH3OCH2. Torsions in transition state are found to be significantly coupled to generate five conformations whose contributions considerably influence the rate constant predictions and are included in the total partition functions via the multi-structural treatment. Variational effects on computed rate constants are observed to be negligibly small and tunneling effect quickly becomes insignificant with increasing temperature. Finally, the four-parameter Arrhenius expression 8.50×1011(T/300)0.58 exp[−3660.61(T−22.27)/(T2+22.272)] cm3mol−1s−1 was used to describe the temperature dependences of the M06-2X/MG3S rate constants evaluated by MS-CVT including small-curvature tunneling correction over the entire temperature range.