An ab initio/Rice-Ramsperger-Kassel-Marcus study of the hydrogen-abstraction reactions of methyl ethers, H3COCH3−x(CH3)x, x = 0–2, by ˙OH; mechanism and kinetics

Abstract

A theoretical study of the mechanism and kinetics of the H-abstraction reaction from dimethyl (DME), ethylmethyl (EME) and iso-propylmethyl (IPME) ethers by the OH radical has been carried out using the high-level methods CCSD(T)/CBS, G3 and G3MP2BH&H. The computationally less-expensive methods of G3 and G3MP2BH&H yield results for DME within 0.2-0.6 and 0.7-0.9 kcal mol(-1), respectively, of the coupled cluster, CCSD(T), values extrapolated to the basis set limit. So the G3 and G3MP2BH&H methods can be confidently used for the reactions of the higher ethers. A distinction is made between the two different kinds of H-atoms, classified as in/out-of the symmetry plane, and it is found that abstraction from the out-of-plane H-atoms proceeds through a stepwise mechanism involving the formation of a reactant complex in the entrance channel and product complex in the exit channel. The in-plane H-atom abstractions take place through a more direct mechanism and are less competitive. Rate constants of the three reactions have been calculated in the temperature range of 500-3000 K using the Variflex code, based on the weak collision, master equation/microcanonical variational RRKM theory including tunneling corrections. The computed total rate constants (cm(3) mol(-1) s(-1)) have been fitted as follows: k(DME) = 2.74 xT(3.94) exp (1534.2/T), k(EME) = 20.93 xT(3.61) exp (2060.1/T) and k(IPME) = 0.55 xT(3.93) exp (2826.1/T). Expressions of the group rate constants for the three different carbon sites are also provided.

Theoretical Chemical Kinetic Study of the H-Atom Abstraction Reactions from Aldehydes and Acids by Ḣ Atoms and ȮH, HȮ2, and ĊH3 Radicals

“…Conventional transition state theory with an asymmetric Eckart tunneling correction has been used to calculate the high-pressure limit rate constants for the title reactions, in the temperature range from 500 to 2000 K. As in our previous works, [3][4][5][6][7][8][9][10][11] complexes have been found in the entrance and exit channels when aldehydes and acids react withȮH, HȮ 2 andĊH 3 radicals. However, when reacting withḢ atoms the formation of the reactant complexes was not observed in the entrance channel where aldehydes and acids react directly with the radical via the transition state to undergo abstraction of a hydrogen atom.…”

Experimental and kinetic modeling study of diethyl ether flames

“…1 ) to obtain reliable rate constants and branching ratios, while those for Habstractions by OH are based on theoretical work by Zhou et al . [26] . Note that the Yasunaga model [5] also used the kinetic data from [26] , but with an erroneous sign for the activation energy of channel I (4040 cal/mol instead of -4040 cal/mol).…”

“…[26] . Note that the Yasunaga model [5] also used the kinetic data from [26] , but with an erroneous sign for the activation energy of channel I (4040 cal/mol instead of -4040 cal/mol). Rate constants of H-abstractions by H-atom calculated here are about two times slower at 1200 K than those in the Yasunaga model [5] .…”

Optimized reaction mechanism rate rules for ignition of normal alkanes

“…In this concept, the fuel specific oxidation steps are described by classes of reactions with the assigned rate constants. Reaction rate constants can be determined from quantum chemistry calculations [14][15][16][17][18] or experimental measurements [19][20][21]. However, while this is practical and desirable for several very important reactions, it is experimentally and computationally difficult to determine the rate constants of all involved reactions due to their large number.…”