Predictive Combustion Kinetics of OH Radical Reactions with a C5 Unsaturated Alcohol: The Competitive H-Abstraction and OH-Addition Reactions of 2-Methyl-3-buten-2-ol

2-Methyl-3-buten-2-ol (MBO232) is a potential biofuel and renewable fuel additive. In a combustion environment, the consumption of MBO232 is mainly through the reaction with a OH radical, one of the most important oxidants. Here, we predict the intricate reactions of MBO232 and OH radicals under a broad range of combustion conditions, that is, 230-2500 K and 0.01-1000 atm. The potential energy surfaces of H-abstraction and OH-addition have been investigated at the CCSD(T)/CBS//M06-2X/def2-TZVP level, and the rate constants were calculated via Rice-Ramsperger-Kassel-Marcus/master equation (RRKM/ME) theory. The decomposition reactions of the critical intermediates from the OH-addition reactions have also been studied. Our results show that OH-addition reactions are dominant below 850 K, while H-abstraction reactions, especially the channel-abstracting H atoms from the methyl groups, are more competitive at higher temperatures. We found that it is necessary to discriminate H atoms attached to the same C atom, as their abstraction rates can differ by up to 1 order of magnitude. The calculated results show good agreement with the reported experimental data. We have provided the modified Arrhenius expressions for rate constants of the dominant channels. The kinetic data determined in this work are of much value for constructing the combustion models of MBO232 and understanding the combustion kinetics and mechanism of other unsaturated alcohols.

Cyclic Ketones as Future Fuels: Reactivity with OH Radicals

For a sustainable energy future, research directions should orient toward exploring new fuels suitable for future advanced combustion engines to achieve better engine efficiency and significantly less harmful emissions. Cyclic ketones, among bio-derived fuels, are of significant interest to the combustion community for several reasons. As they possess high resistance to autoignition characteristics, they can potentially be attractive for fuel blending applications to increase engine efficiency and also to mitigate harmful emissions. Despite their importance, very few studies are rendered in understanding of the chemical kinetic behavior of cyclic ketones under engine-relevant conditions. In this work, we have conducted an experimental investigation for the reaction kinetics of OH radicals with cyclopentanone and cyclohexanone for the first time over a wide range of experimental conditions ( T = 900-1330 K and p ≈ 1.2 bar) in a shock tube. Reaction kinetics was followed by monitoring UV laser absorption of OH radicals near 306.7 nm. Our measured rate coefficients, with an overall uncertainty (2σ) of ±20%, can be expressed in Arrhenius form as (in units of cm³ molecule^-1 s^-1): k1(CPO+OH)=1.20×10-10exp(-2115KT) (902-1297 K); k2(CHO+OH)=2.11×10-10exp(-2268KT) (935-1331 K). Combining our measured data with the single low-temperature literature data, the following three-parameter Arrhenius expressions (in units of cm³ molecule^-1 s^-1) are obtained over a wider temperature range: k₁(CPO + OH) = 1.07×10-13(T300K)3.20exp(1005.7KT) (298-1297 K); k2(CHO+OH)=3.12×10-13(T300K)2.78exp(897.5KT) (298-1331 K). Discrepancies between the theoretical and current experimental results are observed. Earlier theoretical works are found to overpredict our measured rate coefficients. Interestingly, these cyclic ketones exhibit similar reactivity behavior to that of their linear ketone counterparts over the experimental conditions of this work.

Rapid Acceleration of Hydrogen Atom Abstraction Reactions of OH at Very Low Temperatures through Weakly Bound Complexes and Tunneling

A generally accepted principle of chemical kinetics is that a reaction will be very slow at low temperatures if there is an activation barrier on the potential energy surface to form products. However, this Account shows that the reverse is true for gas-phase hydrogen abstraction reactions of the hydroxyl radical, OH, with organic molecules with which it can form a weakly bound (5-30 kJ mol^-1) hydrogen-bonded complex. For hydrogen atom abstraction reactions of OH with volatile organic compounds (VOCs) containing alcohol, ether, carbonyl, and ester functional groups, the reaction accelerates rapidly at very low temperatures, with rate coefficients, k, that can be up to a 1000 times faster than those at room temperature, despite the barrier to products. The OH radical is a crucial intermediate in Earth's atmosphere, combustion processes, and the chemistry of the interstellar medium, where temperatures can reach as low as 10 K, so this behavior has very important implications for gas-phase chemistry in space. The key point is that at low temperatures the lifetime of the OH-VOC complex against re-dissociation back to reactants becomes much longer, and hence the probability of quantum mechanical tunneling under the reaction barrier to form products becomes much higher. These observations were made possible by using Laval nozzles to generate uniform supersonic flows at extremely low temperatures so that condensation of the reagents at reactor walls is avoided. In this Account, the use of laser flash-photolysis combined with laser-induced fluorescence spectroscopy within Laval flows is described to study the unusual kinetics of this type of reaction at temperatures down to 21 K and demonstrate the rapid upturn in the rate coefficient. For the reaction of OH with CH₃OH, further evidence for the precomplex and tunneling mechanism comes from observation of the CH₃O reaction product at very low temperatures, despite it being formed over the higher barrier to reaction. The experimental observations are supported by theoretical calculations using the MESMER master equation package to calculate k( T) and product yields as a function of temperature and which make use of potential energy surfaces determined using ab initio methods. The CH₃O product is formed over a narrower barrier with a larger imaginary frequency and is calculated to be the sole product at very low temperatures. The kinetics of the OH reaction with CH₃OH were measured to be independent of pressure, consistent with a tunneling mechanism rather than any collisional stabilization of the prereactive complex. In this Account, we collate available kinetic data and show that this newly discovered mechanism for H atom transfer reactions appears to be generally applicable for reactions of OH with organic molecules containing oxygenated functional groups, which have been observed in space by radio-astronomy. Rather than being ignored for a range of interstellar environments, these OH reactions are now being included in chemical networks in space and have been shown to significantly influence the abundance of OH, the organic molecules themselves, and reaction products and provide novel routes to forming even more complex functional groups, for example, precursors to prebiotic molecules.

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.

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

Branching ratios of the propene + OH reaction are determined by measuring the rate coefficients of the reaction of OH with propene and five deuterated isotopes of propene.