Comparisons of CBS‐q and G2 calculations on thermodynamic properties, transition states, and kinetics of dimethyl‐ether + O2 reaction system

Abstract

Reaction pathways and kinetics are analyzed on the CH3OC·H2 + O2 reaction system using thermodynamic properties (ΔH°f 298, S°298, and Cp(T)'s 300 ≤ T/K ≤ 1500) derived by two composite ab initio calculation methods, CBS-q and G2. Thermodynamic properties are determined for reactants, intermediate radicals, and transition-state (TS) species and kinetic parameters are determined. Enthalpies of formation (ΔH°f 298 in kcal/mol) of reactant CH3OC·H2 and two intermediate radicals CH3OCH2OO· and C·H2OCH2OOH are determined using isodesmic reactions, where zero-point vibrational energies and thermal corrections to 298.15 K are taken into account. Entropy (S°298 in cal/mol K) and heat capacities (Cp(T)'s, 300 ≤ T/K ≤ 1500, in cal/mol K) are determined using geometric parameters and vibrational frequencies obtained at the MP2(full)/6-31G(d,p) level of theory for CBS-9. Quantum Rice-Ramsperger-Kassel (QRRK) analysis is used to calculate energy-dependent rate constants, k(E) and master equation is used to account for collisional stabilization of adduct and isomer. Overall reaction parameters are determined as a function of temperature and pressure. The dimethyl-ether radical CH3OC·H2 (ΔH°f 298(CBS-q) = 0.1 kcal/mol and ΔH°f 298(G2) = −0.1 kcal/mol) adds to O2 to form a peroxy radical CH3OCH2OO· (ΔH°f 298(CBS-q) = −33.9 kcal/mol and ΔH°f 298(G2) = −34.1 kcal/mol). The peroxy radical can undergo dissociation back to reactants or isomerize via hydrogen shift (TS1) (Ea,rxn(CBS-q) = 17.7 kcal/mol and Ea,rxn(G2) = 20.1 kcal/mol) to form a hydroperoxy alkyl radical C·H2OCH2OOH (ΔH°f 298(CBS-q) = −26.5 kcal/mol and ΔH°f 298(G2) = −25.9 kcal/mol). This alkyl radical can undergo β-scission reaction to formaldehyde (CH2O) + hydroperoxy methyl radical (TS2) C·H2OOH (Ea,rxn(CBS-q) = 24.7 kcal/mol and Ea,rxn(G2) = 25.2 kcal/mol). The hydroperoxy methyl radical rapidly decomposes to a second CH2O + OH. The reaction barriers for CH3OCH2 + O2 to 2 CH2O + OH are less than the energy needed for reaction back to CH3OC·H2 + O2 and provide a low-energy chain propagation path for dimethyl oxidation. OH + CH3OCH3 → CH3OC·H2 + H2O (i) CH3OC·H2 + O2 → 2 CH2O + OH (ii) CH3OCH3 + O2 → 2 CH2O +H2O (i+ii)Comparison of calculated fall-off with experiment indicates that the CBS-q and G2 calculated Ea,rxn for the second transition state (β-scission reaction to CH2O + C·H2OOH) needs to be lowered by 3.3 and 6.0 kcal/mol, respectively, in order to match the data of Sehested et al. Reaction of C·H2OCH2OOH to dioxetane + OH has a barrier 4.6 and 3.4–5.2 kcal/mol above reactant for CBS-q and G2 calculation methods, respectively, and is not important at low temperature. Rate constants of important reactions based on CBS-q results in the form (k = A(T/K)nexp(−Ea/RT), A in cm3/(mol s), Ea in kcal/mol) are k1, (2.33 × 1063)(T/K)−16.89e−11.89/RT for CH3OC·H2 + O2 → CH3OCH2OO·; k3, (6.42 × 1029)(T/K)−5.46e−8.59/RT for CH3OC·H2 + O2 → CH2O + CH2O + OH. © 2000 John Wiley & Sons, Inc. Int J Chem Kinet 32: 435–452, 2000