The effect of hydrogen bonding on the reactivity of OH radicals with prenol and isoprenol: a shock tube and multi-structural torsional variational transition state theory study.

The presence of two functional groups (OH and double bond) in C5 methyl-substituted enols (i.e., isopentenols), such as 3-methyl-2-buten-1-ol (prenol) and 3-methyl-3-buten-1-ol (isoprenol), makes them excellent biofuel candidates as fuel additives. As OH radicals are abundant in both combustion and atmospheric environments, OH-initiated oxidation of these isopentenols over wide ranges of temperatures and pressures needs to be investigated. In alkenes, OH addition to the double bond is prominent at low temperatures (i.e., below ∼700 K), and H-atom abstraction dominates at higher temperatures. However, we find that the OH-initiated oxidation of prenol and isoprenol displays a larger role for OH addition at higher temperatures. In this work, the reaction kinetics of prenol and isoprenol with OH radicals was investigated over the temperature range of 900-1290 K and pressure of 1-5 atm by utilizing a shock tube and OH laser diagnostic. To rationalize these chemical systems, variational transition state theory calculations with multi-structural torsional anharmonicity and small curvature tunneling corrections were run using a potential energy surface characterized at the UCCSD(T)/jun-cc-pVQZ//M06-2X/6-311++G(2df,2pd) level of theory. A good agreement was observed between the experiment and theory, with both predicting a non-Arrhenius behavior and negligible pressure effects. OH additions to the double bond of prenol and isoprenol were found to be important, with at least 50% contribution to the total rate constants even at temperatures as high as 700 and 2000 K, respectively. This behavior was attributed to the stabilizing effect induced by hydrogen bonding between the reacting OH radical and the OH functional group of isopentenols at the saddle points. These stabilizing intermolecular interactions help mitigate the entropic effects that hinder association reactions as temperature increases, thus extending the prominent role of addition pathways to high temperatures. The site-specific rate constants were also found to be slower than their analogous reactions of OH + n-alkenes.

Insights into the Reactions of Hydroxyl Radical with Diolefins from Atmospheric to Combustion Environments.

Hydroxyl radicals and olefins are quite important from a combustion and an atmospheric chemistry standpoint. Large amounts of olefinic compounds are emitted into the earth's atmosphere from both biogenic and anthropogenic sources. Olefins make a significant share in transportation fuels (e.g., up to 20% by volume in gasoline), and they appear as important intermediates during hydrocarbon oxidation. As olefins inhibit low-temperature heat release, they have caught some attention for their applicability in future advanced combustion engine technology. Despite their importance, the literature data for the reactions of olefins are quite scarce. In this work, we have measured the rate coefficients for the reaction of hydroxyl radicals (OH) with several diolefins, namely 1,3-butadiene, cis-1,3-pentadiene, trans-1,3-pentadiene, and 1,4-pentadiene, over a wide range of experimental conditions ( T = 294-468 K and p ∼ 53 mbar; T = 881-1348 K and p ∼ 1-2.5 bar). We obtained the low- T data in a flow reactor using laser flash photolysis and laser-induced fluorescence (LPFR/LIF), and the high- T data were obtained with a shock tube and UV laser-absorption (ST/LA). At low temperatures, we observed differences in the reactivity of cis- and trans-1,3-pentadiene, but these molecules exhibited similar reactivity at high temperatures. Similar to monoolefins + OH reactions, we observed negative temperature dependence for dienes + OH reactions at low temperatures-revealing that OH-addition channels prevail at low temperatures. Except for the 1,4-pentadiene + OH reaction, which shows evidence of significant H-abstraction reactions even at low-temperatures, other diolefins studied here almost exclusively undergo addition reaction with OH radicals at the low-temperature end of our experiments; whereas the reactions mainly switch to hydrogen abstraction at high temperatures. These reactions show complex Arrhenius behavior as a result of many possible chemical pathways in such a convoluted system.

Theoretical and Shock Tube Study of the Rate Constants for Hydrogen Abstraction Reactions of Ethyl Formate.

We report a systematic chemical kinetics study of the H atom abstractions from ethyl formate (EF) by H, O(3P), CH3, OH, and HO2 radicals. The geometry optimization and frequency calculation of all the species were conducted using the M06 method and the cc-pVTZ basis set. The one-dimensional hindered rotor treatment of the reactants and transition states and the intrinsic reaction coordinate analysis were also performed at the M06/cc-pVTZ level of theory. The relative electronic energies were calculated at the CCSD(T)/cc-pVXZ (where X = D, T) level of theory and further extrapolated to the complete basis set limit. Rate constants for the tittle reactions were calculated over the temperature range 500-2500 K by the transition state theory (TST) in conjunction with the asymmetric Eckart tunneling effect. In addition, the rate constants of H-abstraction by hydroxyl radical were measured in shock tube experiments at 900-1321 K and 1.4-2.0 atm. Our theoretical rate constants of OH + EF → products agree well with the experimental results within 15% over the experimental temperature range of 900-1321 K. Branching ratios for the five types of H-abstraction reactions were also determined from their individual site-specific rate constants.

A combined high-temperature experimental and theoretical kinetic study of the reaction of dimethyl carbonate with OH radicals.

The reaction kinetics of dimethyl carbonate (DMC) and OH radicals were investigated behind reflected shock waves over the temperature range of 872-1295 K and at pressures near 1.5 atm. Reaction progress was monitored by detecting OH radicals at 306.69 nm using a UV laser absorption technique. The rate coefficients for the reaction of DMC with OH radicals were extracted using a detailed kinetic model developed by Glaude et al. (Proc. Combust. Inst. 2005, 30(1), 1111-1118). The experimental rate coefficients can be expressed in Arrhenius form as: kexpt'l = 5.15 × 1013 exp(-2710.2/T) cm3 mol-1 s-1. To explore the detailed chemistry of the DMC + OH reaction system, theoretical kinetic analyses were performed using high-level ab initio and master equation/Rice-Ramsperger-Kassel-Marcus (ME/RRKM) calculations. Geometry optimization and frequency calculations were carried out at the second-order Møller-Plesset (MP2) perturbation level of theory using Dunning's augmented correlation consistent-polarized valence double-ζ basis set (aug-cc-pVDZ). The energy was extrapolated to the complete basis set using single point calculations performed at the CCSD(T)/cc-pVXZ (where X = D, T) level of theory. For comparison purposes, additional ab initio calculations were also carried out using composite methods such as CBS-QB3, CBS-APNO, G3 and G4. Our calculations revealed that the H-abstraction reaction of DMC by OH radicals proceeds via an addition elimination mechanism in an overall exothermic process, eventually forming dimethyl carbonate radicals and H2O. Theoretical rate coefficients were found to be in excellent agreement with those determined experimentally. Rate coefficients for the DMC + OH reaction were combined with literature rate coefficients of four straight chain methyl ester + OH reactions to extract site-specific rates of H-abstraction from methyl esters by OH radicals.

H-Abstraction by OH from Large Branched Alkanes: Overall Rate Measurements and Site-Specific Tertiary Rate Calculations.

Reaction rate coefficients for the reaction of hydroxyl (OH) radicals with nine large branched alkanes (i.e., 2-methyl-3-ethyl-pentane, 2,3-dimethyl-pentane, 2,2,3-trimethylbutane, 2,2,3-trimethyl-pentane, 2,3,4-trimethyl-pentane, 3-ethyl-pentane, 2,2,3,4-tetramethyl-pentane, 2,2-dimethyl-3-ethyl-pentane, and 2,4-dimethyl-3-ethyl-pentane) are measured at high temperatures (900-1300 K) using a shock tube and narrow-line-width OH absorption diagnostic in the UV region. In addition, room-temperature measurements of six out of these nine rate coefficients are performed in a photolysis cell using high repetition laser-induced fluorescence of OH radicals. Our experimental results are combined with previous literature measurements to obtain three-parameter Arrhenius expressions valid over a wide temperature range (300-1300 K). The rate coefficients are analyzed using the next-nearest-neighbor (N-N-N) methodology to derive nine tertiary (T003, T012, T013, T022, T023, T111, T112, T113, and T122) site-specific rate coefficients for the abstraction of H atoms by OH radicals from branched alkanes. Derived Arrhenius expressions, valid over 950-1300 K, are given as (the subscripts denote the number of carbon atoms connected to the next-nearest-neighbor carbon): T003 = 1.80 × 10-10 exp(-2971 K/T) cm3 molecule-1 s-1; T012 = 9.36 × 10-11 exp(-3024 K/T) cm3 molecule-1 s-1; T013 = 4.40 × 10-10 exp(-4162 K/T) cm3 molecule-1 s-1; T022 = 1.47 × 10-10 exp(-3587 K/T) cm3 molecule-1 s-1; T023 = 6.06 × 10-11 exp(-3010 K/T) cm3 molecule-1 s-1; T111 = 3.98 × 10-11 exp(-1617 K/T) cm3 molecule-1 s-1; T112 = 9.08 × 10-12 exp(-3661 K/T) cm3 molecule-1 s-1; T113 = 6.74 × 10-9 exp(-7547 K/T) cm3 molecule-1 s-1; T122 = 3.47 × 10-11 exp(-1802 K/T) cm3 molecule-1 s-1.

Rate Coefficients of the Reaction of OH with Allene and Propyne at High Temperatures.

Allene (H2C═C═CH2; a-C3H4) and propyne (CH3C≡CH; p-C3H4) are important species in various chemical environments. In combustion processes, the reactions of hydroxyl radicals with a-C3H4 and p-C3H4 are critical in the overall fuel oxidation system. In this work, rate coefficients of OH radicals with allene (OH + H2C═C═CH2 → products) and propyne (OH + CH3C≡CH → products) were measured behind reflected shock waves over the temperature range of 843-1352 K and pressures near 1.5 atm. Hydroxyl radicals were generated by rapid thermal decomposition of tert-butyl hydroperoxide ((CH3)3-CO-OH), and monitored by narrow line width laser absorption of the well-characterized R1(5) electronic transition of the OH A-X (0,0) electronic system near 306.7 nm. Results show that allene reacts faster with OH radicals than propyne over the temperature range of this study. Measured rate coefficients can be expressed in Arrhenius form as follows: kallene+OH(T) = 8.51(±0.03) × 10-22T3.05 exp(2215(±3)/T), T = 843-1352 K; kpropyne+OH(T) = 1.30(±0.07) × 10-21T3.01 exp(1140(±6)/T), T = 846-1335 K.

Modeling Ignition of a Heptane Isomer: Improved Thermodynamics, Reaction Pathways, Kinetics, and Rate Rule Optimizations for 2-Methylhexane.

Accurate chemical kinetic combustion models of lightly branched alkanes (e.g., 2-methylalkanes) are important to investigate the combustion behavior of real fuels. Improving the fidelity of existing kinetic models is a necessity, as new experiments and advanced theories show inaccuracies in certain portions of the models. This study focuses on updating thermodynamic data and the kinetic reaction mechanism for a gasoline surrogate component, 2-methylhexane, based on recently published thermodynamic group values and rate rules derived from quantum calculations and experiments. Alternative pathways for the isomerization of peroxy-alkylhydroperoxide (OOQOOH) radicals are also investigated. The effects of these updates are compared against new high-pressure shock tube and rapid compression machine ignition delay measurements. It is shown that rate constant modifications are required to improve agreement between kinetic modeling simulations and experimental data. We further demonstrate the ability to optimize the kinetic model using both manual and automated techniques for rate parameter tunings to improve agreement with the measured ignition delay time data. Finally, additional low temperature chain branching reaction pathways are shown to improve the model's performance. The present approach to model development provides better performance across extended operating conditions while also strengthening the fundamental basis of the model.

Correction: A shock tube study of the branching ratios of propene + OH reaction.

Correction for 'A shock tube study of the branching ratios of propene + OH reaction' by Jihad Badra et al., Phys. Chem. Chem. Phys., 2015, 17, 2421-2431.

A shock tube study of the branching ratios of propene + OH reaction.

Absolute rate coefficients for the reaction of the OH radical with propene (C3H6) and five deuterated isotopes, propene-1-D1 (CDHCHCH3), propene-1,1-D2 (CD2CHCH3), propene-1,1,2-D3 (CD2CDCH3), propene-3,3,3-D3 (CH2CHCD3), and propene-D6 (C3D6), were measured behind reflected shock waves over the temperature range of 818-1460 K and pressures near 1 atm. The reaction progress was followed by monitoring the OH radical near 306.7 nm using UV laser absorption. Kinetic isotope effects in the measured rate coefficients are discussed and rationalized for the site-specific H-abstraction by the OH radical. The first experimental measurements for the branching ratio of the title reaction are reported and compared with transition state theory calculations. The allylic H-atom abstraction of propene by OH radicals was found to be the most dominant reaction pathway followed by propen-1-yl and propen-2-yl channels over the entire temperature range of this study. The derived Arrhenius expressions for various site-specific rate coefficients over 818-1442 K are (the subscript in the rate coefficient identifies the position of H or D atom according to the IUPAC nomenclature of alkenes):