Ab Initio Reaction Kinetics of Hydrogen Abstraction from Methyl Formate by Hydrogen, Methyl, Oxygen, Hydroxyl, and Hydroperoxy Radicals

Abstract

Combustion of renewable biofuels, including energy-dense biodiesel, is expected to contribute significantly toward meeting future energy demands in the transportation sector. Elucidating detailed reaction mechanisms will be crucial to understanding biodiesel combustion, and hydrogen abstraction reactions are expected to dominate biodiesel combustion during ignition. In this work, we investigate hydrogen abstraction by the radicals H·, CH(3)·, O·, HO(2)·, and OH· from methyl formate, the simplest surrogate for complex biodiesels. We evaluate the H abstraction barrier heights and reaction enthalpies, using multireference correlated wave function methods including size-extensivity corrections and extrapolation to the complete basis set limit. The barrier heights predicted for abstraction by H·, CH(3)·, and O· are in excellent agreement with derived experimental values, with errors ≤1 kcal/mol. We also predict the reaction energetics for forming reactant complexes, transition states, and product complexes for reactions involving HO(2)· and OH·. High-pressure-limit rate constants are computed using transition state theory within the separable-hindered-rotor approximation for torsions and the harmonic oscillator approximation for other vibrational modes. The predicted rate constants differ significantly from those appearing in the latest combustion kinetics models of these reactions.

Variational transition state theory rate constants and H/D kinetic isotope effects for CH3 + CH3OCOH reactions

“…Considering that in general, the MP2 theory usually predicts high values of mean unsigned error (4.3 kcal/mol) of barrier height, the calculated barriers for both R1 and R2 with MP2/aug‐cc‐pVDZ indicate a favorable consistent error cancellation, as observed in our previous study of the Cl + C 2 H 6 hydrogen abstraction reaction . For R2, our present benchmark results for the classical barrier height ( V ‡ ) and electronic energy ( ∆E ) kcal/mol) are in close agreement with those ones obtained with the MRSDCI+DS approach, and also they are close to the QCISD(T) results . All high‐level calculations predict values of ( V ‡ ) which are in the range of 14.9–15.2 kcal/mol, and for calculations of ( ∆E ), the values are in the range of −4.4 to −4.7 kcal/mol, respectively.…”

“…Those thermochemical data are in good agreement with the computed values of of −5.5 (R1) and − 4.7 (R2) computed with the CBS results. The CBS D‐T and CBS T‐Q best calculations give to R1 and R2 values of in better agreement with experiment, and they also predict classical barrier height within the same range of MRACPF/cc‐pV∞Z calculations, thus they can be considered a well‐balanced approach for the energetic properties of these reaction paths.…”

“…The Cartesian coordinates of the optimized geometries, symmetry group point, electronic energies, and the zero‐point vibrational energy (ZPVE) are available as Appendix S1. Table gives the values of classical barrier height ( V ‡ ), electronic energy ( ∆E ) kcal/mol) and enthalpy of reaction at 0 K () of R1 and R2 paths, as well a previous calculation with the MRACPF/cc‐pV∞Z method . Calculations performed with the M06‐2X, BB1K, MP2, CCSD(T), and CBS methods predict, in general, that the classical barrier height of R1 is smaller than the value of R2 by 1.0 to 1.5 kcal/mol.…”

“…For R1, the values of benchmark calculations of ( ∆E ) (in parenthesis) are CCSD(T)/maug‐cc‐pVQZ (−6.2 kcal/mol), CBS D‐T (−6.3 kcal/mol) and CBS T‐Q (−6.2 kcal/mol); the low‐level methods that give energetic values in better agreement with the CBS best calculations were obtained at the M06‐2X/aug‐cc‐pVTZ (−6.3 kcal/mol), MP2/cc‐pVTZ (−6.3 kcal/mol), and MP2/aug‐cc‐pVTZ (−5.9 kcal/mol) levels. For the classical barrier height, the best agreement (~1.0 kcal/mol) between the calculations computed by the MRSDCI+DS (14.7 kcal/mol), CBS D‐T (13.2 kcal/mol), and CBS T‐Q (13.5 kcal/mol) methods was achieved at the MP2 level with the aug‐cc‐pVDZ, cc‐pVTZ, and aug‐cc‐pVTZ basis sets with values of 14.3, 15.2, and 14.7 kcal/mol, respectively.…”

“…Rate constants are calculated with the transition state theory (TST) for which one‐dimensional tunneling transmissions coefficients were included using the method proposed by Wigner . Tan et al have used the B3LYP/cc‐pVTZ method to calculate the equilibrium geometries, harmonic frequencies, and the intrinsic reaction coordinate paths connecting the saddle point to reagents and products. The rate constants were calculated by the conventional TST with tunneling corrections included at the level of one‐dimensional asymmetric Eckart approximation …”