Products from the oxidation of linear isomers of hexene

An experimental and master-equation modeling study of the kinetics of the reaction between resonance-stabilized (CH3)2CCHCH2 radical and molecular oxygen

The kinetics of the reaction between resonance-stabilized (CH₃)₂CCHCH₂ radical (R) and O₂ has been investigated using photoionization mass spectrometry, and master equation (ME) simulations were performed to support the experimental results. The kinetic measurements of the (CH₃)₂CCHCH₂ + O₂ reaction (1) were carried out at low helium bath-gas pressures (0.2-5.7 Torr) and over a wide temperature range (238-660 K). Under low temperature (238-298 K) conditions, the pressure-dependent bimolecular association reaction R + O₂ → ROO determines kinetics, until at an intermediate temperature range (325-373 K) the ROO adduct becomes thermally unstable and increasingly dissociates back to the reactants with increasing temperature. The initial association of O₂ with (CH₃)₂CCHCH₂ radical occurs on two distinct sites: terminal 1(t) and non-terminal 1(nt) sites on R, leading to the barrierless formation of ROO_(t) and ROO_(nt) adducts, respectively. Important for autoignition modelling of olefinic compounds, bimolecular reaction channels appear to open for the R + O₂ reaction at high temperatures (T > 500 K) and pressure-independent bimolecular rate coefficients of reaction (1) with a weak positive temperature dependence, (2.8-4.6) × 10^-15 cm³ molecule^-1 s^-1, were measured in the temperature range of 500-660 K. At a temperature of 501 K, a product signal of reaction (1) was observed at m/z = 68, probably originating from isoprene. To explore the reaction mechanism of reaction (1), quantum chemical calculations and ME simulations were performed. According to the ME simulations, without any adjustment to energies, the most important and second most important product channels at the high temperatures are isoprene + HO₂ (yield > 91%) and (2R/S)-3-methyl-1,2-epoxybut-3-ene + OH (yield < 8%). After modest adjustments to ROO_(t) and ROO_(nt) well-depths (∼0.7 kcal mol^-1 each) and barrier height for the transition state associated with the kinetically most dominant channel, R + O₂ → isoprene + HO₂ (∼2.2 kcal mol^-1), the ME model was able to reproduce the experimental findings. Modified Arrhenius expressions for the kinetically important reaction channels are enclosed to facilitate the use of current results in combustion models.

Microwave spectroscopic detection of flame-sampled combustion intermediates

Microwave spectroscopy was used to detect and identify combustion intermediates after sampling out of laboratory-scale model flames.

Thermochemistry and Kinetic Analysis of the Unimolecular Oxiranyl Radical Dissociation Reaction: A Theoretical Study

Oxirane structures are important in organic synthesis, and they are important initial products in the oxidation reactions of alkyl radicals. The thermochemical properties (enthalpy of formation, entropy, and heat capacity) for the reaction steps of the unimolecular oxiranyl radical dissociation reaction are determined and compared with the available literature. The overall ring opening and subsequent steps involve four types of reactions: β-scission ring opening, intramolecular hydrogen transfer, β-scission hydrogen elimination, and β-scission methyl radical elimination. The enthalpies of formation of the transition states are determined and evaluated using six popular Density Functional Theory (DFT) calculation methods (B3LYP, B2PLYP, M06, M06-2X, ωB97X, ωB97XD), each combined with three different basis sets. The DFT enthalpy values are compared with five composite calculation methods (G3, G4, CBS-QB3, CBS-APNO, W1U), and by CCSD(T)/aug-cc-pVTZ. Kinetic parameters are determined versus pressure and temperature for the unimolecular dissociation pathways of an oxiranyl radical, which include the chemical activation reactions of the ring-opened oxiranyl radical relative to the ring-opening barrier. Multifrequency quantum Rice Ramsperger Kassel (QRRK) analysis is used to determine k(E) with master equation analysis for falloff. The major overall reaction pathway at lower combustion temperatures is oxiranyl radical dissociation to a methyl radical and carbon monoxide. Oxiranyl radical dissociation to a ketene and hydrogen atom is the key reaction path above 700 K.

Metal ion hydrocarbon bidentate bonding in alkyl acetates, methyl alkanoates, alcohols and 1-alkenes: a comparative study

The relative affinity of the monovalent metal ions Li⁺, Na⁺, Cu⁺ and Ag⁺ towards a series of aliphatic alkyl acetates and some selected 1-alkenes (P) was examined using the kinetic method. A detailed analysis of the dissociation characteristics of a series of mixed metal-bound dimer ions of the type P₁-M⁺-P₂ and the evaluated proton affinities (PAs) of the monomers shows that the affinity of the cation towards long-chain alkyl acetates and alkenes (having a chain length ≤ C₄) is markedly enhanced. In line with recent studies of nitriles, alcohols and methyl alkanoates, this is attributed to a bidentate interaction of the metal ion with the functional group or double bond and the aliphatic chain. In particular, the longer chain alkyl acetates, methyl alkanoates and alcohols show a remarkably similar behaviour with respect to silver ion hydrocarbon bonding. The Ag⁺ adducts of the alkyl acetates dissociate by loss of CH₃COOH. This reaction becomes more pronounced at longer chain lengths, which points to metal ion bidentate formation in [Ag⁺···1-alkene] product ions having a long hydrocarbon chain. In the same vein, the heterodimers [1- hexene···Ag⁺···1-heptene] and [1- heptene···Ag⁺···1-octene] dissociate primarily into [Ag⁺···1-heptene] and [Ag⁺···1-octene] ions, respectively. Hydrocarbon bidentate formation in [Ag⁺···1-octene] also reveals itself by the reluctance of this ion to react with water in an ion trap, as opposed to [Ag⁺···1-hexene] which readily undergoes hydration.

Chemical Kinetic Influences of Alkyl Chain Structure on the High Pressure and Temperature Oxidation of a Representative Unsaturated Biodiesel: Methyl Nonenoate

The high pressure and temperature oxidation of methyl trans-2-nonenoate, methyl trans-3-nonenoate, 1-octene, and trans-2-octene are investigated experimentally to probe the influence of the double bond position on the chemical kinetics of long esters and alkenes. Single pulse shock tube experiments are performed in the ranges p = 3.8-6.2 MPa and T = 850-1500 K, with an average reaction time of 2 ms. Gas chromatographic measurements indicate increased reactivity for trans-2-octene compared to 1-octene, whereas both methyl nonenoate isomers have reactivities similar to that of 1-octene. A difference in the yield of stable intermediates is observed for the octenes when compared to the methyl nonenoates. Chemical kinetic models are developed with the aid of the Reaction Mechanism Generator to interpret the experimental results. The models are created using two different base chemistry submodels to investigate the influence of the foundational chemistry (i.e., C0-C4), whereas Monte Carlo simulations are performed to examine the quality of agreement with the experimental results. Significant uncertainties are found in the chemistry of unsaturated esters with the double bonds located close to the ester groups. This work highlights the importance of the foundational chemistry in predictive chemical kinetics of biodiesel combustion at engine relevant conditions.

Theoretical Investigation on the Reaction between OH Radical and 4,4-Dimethyl-1-pentene in the Presence of O2

The atmospheric oxidation mechanism of 4,4-dimethyl-1-pentene (DMP441) initiated by OH radical has been theoretically investigated at the BH&HLYP/6-311++G(d,p) and CCSD(T)/6-31+G(d,p) levels of theory. HC(O)H and 3,3-dimethylbutanal [(CH3)3CCH2C(O)H] are identified in our calculations as major products in the OH-radical-initiated degradation of DMP441 in the presence of O2. However, the epoxide conformers and enols are expected to be minor products because of the high isomerization barriers involved. The calculated results are in qualitative accordance with experimental evidence. Conventional transition state theory has been used to calculate the rate constants of the initial addition channels of the OH + DMP441 reaction over the temperature range 220-500 K. The computed total rate constant at 298 K is 2.20 × 10(-11) cm(3) molecule(-1) s(-1), which is in very good agreement with the experimental value. Furthermore, it has been found that the calculated rate constant exhibits a weak non-Arrhenius behavior over the temperature range 220-500 K. The computed expression for the rate constant is k(OH+DMP441) = 1.22 × 10(-12) exp[(880 K)/T] cm(3) molecule(-1) s(-1).

Detection and Identification of the Keto-Hydroperoxide (HOOCH2OCHO) and Other Intermediates during Low-Temperature Oxidation of Dimethyl Ether

In this paper we report the detection and identification of the keto-hydroperoxide (hydroperoxymethyl formate, HPMF, HOOCH2OCHO) and other partially oxidized intermediate species arising from the low-temperature (540 K) oxidation of dimethyl ether (DME). These observations were made possible by coupling a jet-stirred reactor with molecular-beam sampling capabilities, operated near atmospheric pressure, to a reflectron time-of-flight mass spectrometer that employs single-photon ionization via tunable synchrotron-generated vacuum-ultraviolet radiation. On the basis of experimentally observed ionization thresholds and fragmentation appearance energies, interpreted with the aid of ab initio calculations, we have identified HPMF and its conceivable decomposition products HC(O)O(O)CH (formic acid anhydride), HC(O)OOH (performic acid), and HOC(O)OH (carbonic acid). Other intermediates that were detected and identified include HC(O)OCH3 (methyl formate), cycl-CH2-O-CH2-O- (1,3-dioxetane), CH3OOH (methyl hydroperoxide), HC(O)OH (formic acid), and H2O2 (hydrogen peroxide). We show that the theoretical characterization of multiple conformeric structures of some intermediates is required when interpreting the experimentally observed ionization thresholds, and a simple method is presented for estimating the importance of multiple conformers at the estimated temperature (∼100 K) of the present molecular beam. We also discuss possible formation pathways of the detected species: for example, supported by potential energy surface calculations, we show that performic acid may be a minor channel of the O2 + ĊH2OCH2OOH reaction, resulting from the decomposition of the HOOCH2OĊHOOH intermediate, which predominantly leads to the HPMF.

Experimental investigation of the low temperature oxidation of the five isomers of hexane

The low-temperature oxidation of the five hexane isomers (n-hexane, 2-methyl-pentane, 3-methyl-pentane, 2,2-dimethylbutane, and 2,3-dimethylbutane) was studied in a jet-stirred reactor (JSR) at atmospheric pressure under stoichiometric conditions between 550 and 1000 K. The evolution of reactant and product mole fraction profiles were recorded as a function of the temperature using two analytical methods: gas chromatography and synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS). Experimental data obtained with both methods were in good agreement for the five fuels. These data were used to compare the reactivity and the nature of the reaction products and their distribution. At low temperature (below 800 K), n-hexane was the most reactive isomer. The two methyl-pentane isomers have about the same reactivity, which was lower than that of n-hexane. 2,2-Dimethylbutane was less reactive than the two methyl-pentane isomers, and 2,3-dimethylbutane was the least reactive isomer. These observations are in good agreement with research octane numbers given in the literature. Cyclic ethers with rings including 3, 4, 5, and 6 atoms have been identified and quantified for the five fuels. While the cyclic ether distribution was notably more detailed than in other literature of JSR studies of branched alkane oxidation, some oxiranes were missing among the cyclic ethers expected from methyl-pentanes. Using SVUV-PIMS, the formation of C2-C3 monocarboxylic acids, ketohydroperoxides, and species with two carbonyl groups have also been observed, supporting their possible formation from branched reactants. This is in line with what was previously experimentally demonstrated from linear fuels. Possible structures and ways of decomposition of the most probable ketohydroperoxides were discussed. Above 800 K, all five isomers have about the same reactivity, with a larger formation from branched alkanes of some unsaturated species, such as allene and propyne, which are known to be soot precursors.