Theoretical study of rate coefficients and branching fractions in the propene + OH reaction

High-level ab initio calculation of the C3H6 + OH potential energy surface was coupled with master equation methods to compute rate coefficients and product branching ratios for temperatures of 300–2500 K. Our model reproduces the available experimental results well. We find a surprisingly wide range of bimolecular product channels for this reaction, including vinyl alcohol, acetaldehyde, allyl radical, acetone, ethene, propanal, propenol and formaldehyde. ∗ Corresponding author: [email protected] Proceedings of the European Combustion Meeting 2009 Introduction Alkenes comprise a substantial fraction of practical fuels. The chemistry of propene combustion is of particular interest because it can serve as a prototype for alkene chemistry and because it is simple enough to be thoroughly analyzed. Propene combustion readily leads to the formation of allyl radicals, one of the resonancestabilized radicals that contribute to soot formation in flames. Experimentally, propene combustion is studied in flames (e.g. Ref. 3), and detailed kinetic mechanisms also exist in the literature (e.g. Refs. 4-6). The overall rate coefficient of the title reaction has been extensively studied experimentally (e.g. Refs. 711) and is reported in data evaluations 2 for a wide temperature range. The addition of the OH radical to the double bond is dominant below ~500 K and has a negative temperature dependence. Above ~700 K abstraction is the principal process, and the temperature dependence is positive. There is only one study on the bimolecular product distribution at low pressures. The reaction is also intimately linked to the OH-initiated oxidation of propanol, 14 and the knowledge of the rate coefficient is important in measurements of hydroxypropene + O2 rate coefficients. There have been theoretical calculations for portions of the underlying C3H7O potential-energy surface (PES). However, there is a lack of accurately calculated detailed rate coefficients and product channels for a wide pressure and temperature range. In this work we calculate the overall rate coefficient and the branching fractions for the 300–2500 K temperature and zero to infinite pressure ranges using high-level ab initio methods and RRKM-based multiwell master equation methods. Due to the limitations of paper length, only selected results are presented here. Construction of the PES Optimized geometries, frequencies, internal rotational potentials, and energies of the stable complexes and saddle points along the intrinsic reaction coordinate (IRC) were calculated on the C3H7O potential energy surface. Geometry optimization and IRC scans were performed with density functional theory (DFT) calculations using the B3LYP functional applying the d,p-polarized split valence 6-311++G(d,p) Gaussian basis set. In all cases lowest energy conformers were searched for in a systematic manner. For the computation of accurate single point energies we used the restricted quadratic-configurationinteraction method with single and double excitations and correction for triple excitations, RQCISD(T), with the cc-pVnZ basis set, n = (T,Q), extrapolated to the infinite basis set limit cc-pV∞Z. The DFT calculations were performed using the Gaussian 03 suite of programs, and other quantum chemical calculations employed the MOLPRO package. Calculation of rate coefficient RRKM/master-equation (ME) methodology developed by Miller and Klippenstein was used to determine rate coefficients from the computed stationary point properties. The low-frequency torsional modes were treated as hindered rotors using the PitzerGwinn approach applied to state densities. The hindering potentials were determined by fitting Fourier series to the B3LYP/6-311++G(d,p) energies along the internal rotation. Asymmetric Eckart barriers were used to model tunneling in 1D. Collisional energy transfer was approximated by a simple exponential-down model, where the average downward transfer parameter 〈∆Ed〉 was temperature-dependent in the form 200×(T/298) cm. Lennard-Jones parameters for the C3H7O isomers were taken as that of propanol, 26 σ = 4.459 Å, ε/kB = 576.7 K. The multiwell ME calculations were carried out with the VARIFLEX program package. The entrance channel of the reaction has a van der Waals well corresponding to the OH roughly midway between the two C-atoms of the double bond in