Concerted Elimination Research Articles

ChemInform Abstract: A Well‐Defined (POCOP)Rh Catalyst for the Coupling of Aryl Halides with Thiols.

This article describes a well-defined pincer-Rh catalyst for C-S cross-coupling reactions. (POCOP)Rh(H)(Cl) serves as an active precatalyst for the coupling of aryl chlorides and bromides with aryl and alkyl thiols under reasonable conditions (3% mol cat., 110 °C, 2-24 h, >90% yield). For select substrates, >90% yields were obtained with catalyst loading as low as 0.1%. Key mechanistic intermediates have been isolated and fully characterized, including (POCOP)Rh(Ph)(SPh) (6a) and (POCOP)Rh(SPh2) (6b). The aryl/bis(phosphinite) (POCOP)Rh system has been shown to favor aryl thiolate reductive elimination at elevated temperatures and in some cases at room temperature, compared with the analogous diarylamido/bis(phosphine) (PNP)Rh pincer system. Concerted reductive elimination has been studied with 6a directly and in the presence of aryl bromide and aryl chloride traps. This investigation demonstrates a clear rate dependence on aryl chloride concentration during catalysis, a dependence that is absent when using aryl bromides. The rate of catalysis is dramatically reduced or brought to zero for ortho-tolyl halides, which can be traced to slower C-S coupling and slower carbon-halogen oxidative addition for ortho-substituted aryls. The influence of the sterics in the thiol component is less straightforward. The S-H oxidative addition product (POCOP)Rh(H)(SPh) (16) has been fully characterized and its reactivity has been examined, resulting in the isolation of the sodium-thiolate adduct (POCOP)Rh(NaSPh) (19). The solid-state structure of 19 shows Na interactions not only with sulfur, but also with a neighboring Rh and the chelating aryl carbon of the pincer framework. The reactivity of 16 and 19 indicates that these potential side products should not hinder catalysis.

A chemical mechanism for low to high temperature oxidation of methylcyclohexane as a component of transportation fuel surrogates

Surrogate fuels consisting of a mixture of well-studied hydrocarbons are often used to model real fuels in typical combustion studies. A major challenge, however, is the capability to design compact and reliable kinetic models that capture all the specificities of the simpler, but still multi-component surrogates. This task is further complicated by the diverse nature of the hydrocarbons commonly considered as potential surrogate components, since they typically result in large detailed reaction schemes. Towards addressing this challenge, the present work proposes a single, compact, and reliable chemical mechanism, that can accurately describe the oxidation of a wide range of fuels, which are important components of surrogate fuels. A well-characterized mechanism appropriate for the oxidation of smaller hydrocarbon species (Blanquart et al., 2009), as well as several substituted aromatic species and n-dodecane (Narayanaswamy et al., 2010, 2014), well suited as a base to model surrogates, has now been extended to describe the oxidation of methylcyclohexane, a representative of the cyclic alkane class, which is often used in jet fuel surrogates. To ensure compactness of the kinetic scheme, a short mechanism for the low to high temperature oxidation of methylcyclohexane is extracted from the detailed scheme of Pitz et al. (2007) and integrated in a systematic way into the previous model. Rate coefficient changes based on recent recommendations from literature, and an additional concerted elimination pathway important at moderate to low temperatures are introduced to the resulting chemical mechanism, which improve the model predictions. Extensive validation of the revised kinetic model is performed using a wide range of experimental conditions and data sets.

A well-defined (POCOP)Rh catalyst for the coupling of aryl halides with thiols.

This article describes a well-defined pincer-Rh catalyst for C-S cross-coupling reactions. (POCOP)Rh(H)(Cl) serves as an active precatalyst for the coupling of aryl chlorides and bromides with aryl and alkyl thiols under reasonable conditions (3% mol cat., 110 °C, 2-24 h, >90% yield). For select substrates, >90% yields were obtained with catalyst loading as low as 0.1%. Key mechanistic intermediates have been isolated and fully characterized, including (POCOP)Rh(Ph)(SPh) (6a) and (POCOP)Rh(SPh2) (6b). The aryl/bis(phosphinite) (POCOP)Rh system has been shown to favor aryl thiolate reductive elimination at elevated temperatures and in some cases at room temperature, compared with the analogous diarylamido/bis(phosphine) (PNP)Rh pincer system. Concerted reductive elimination has been studied with 6a directly and in the presence of aryl bromide and aryl chloride traps. This investigation demonstrates a clear rate dependence on aryl chloride concentration during catalysis, a dependence that is absent when using aryl bromides. The rate of catalysis is dramatically reduced or brought to zero for ortho-tolyl halides, which can be traced to slower C-S coupling and slower carbon-halogen oxidative addition for ortho-substituted aryls. The influence of the sterics in the thiol component is less straightforward. The S-H oxidative addition product (POCOP)Rh(H)(SPh) (16) has been fully characterized and its reactivity has been examined, resulting in the isolation of the sodium-thiolate adduct (POCOP)Rh(NaSPh) (19). The solid-state structure of 19 shows Na interactions not only with sulfur, but also with a neighboring Rh and the chelating aryl carbon of the pincer framework. The reactivity of 16 and 19 indicates that these potential side products should not hinder catalysis.

Understanding the Mechanisms of Unusually Fast HH, CH, and CC Bond Reductive Eliminations from Gold(III) Complexes

Carbon-carbon bond reductive elimination from gold(III) complexes are known to be very slow and require high temperatures. Recently, Toste and co-workers have demonstrated extremely rapid CC reductive elimination from cis-[AuPPh3 (4-F-C6 H4 )2 Cl] even at low temperatures. We have performed DFT calculations to understand the mechanistic pathway for these novel reductive elimination reactions. Direct dynamics calculations inclusive of quantum mechanical tunneling showed significant contribution of heavy-atom tunneling (>25 %) at the experimental reaction temperatures. In the absence of any competing side reactions, such as phosphine exchange/dissociation, the complex cis-[Au(PPh3 )2 (4-F-C6 H4 )2 ](+) was shown to undergo ultrafast reductive elimination. Calculations also revealed very facile, concerted mechanisms for HH, CH, and CC bond reductive elimination from a range of neutral and cationic gold(III) centers, except for the coupling of sp(3) carbon atoms. Metal-carbon bond strengths in the transition states that originate from attractive orbital interactions control the feasibility of a concerted reductive elimination mechanism. Calculations for the formation of methane from complex cis-[AuPPh3 (H)CH3 ](+) predict that at -52 °C, about 82 % of the reaction occurs by hydrogen-atom tunneling. Tunneling leads to subtle effects on the reaction rates, such as large primary kinetic isotope effects (KIE) and a strong violation of the rule of the geometric mean of the primary and secondary KIEs.

Concerted elimination of Br2+ resulting from the Coulomb explosion of 1,2-dibromoethane in an intense femtosecond laser field

Concerted elimination pathway leading to Br2+ and C2H4+ from 1,2-dibromoethane molecule has been investigated in 800nm femtosecond laser field by dc-slice imaging technology. The kinetic energy release and angular distributions of Br2+ and C2H4+ demonstrate that Br2+ results from the two-body Coulomb explosion of the doubly charged parent ion. Ab initio calculations show that the doubly charged precursor overcomes a small energy barrier (0.18eV) and then dissociates into Br2+ and C2H4+ through a synchronous concerted elimination mechanism. Moreover, the relative yield of the Br2+ channel is obtained, and it remains about 3.4% when the laser intensity exceeds 1.0×1014W/cm2.

Theoretical Study on Dissociation Mechanisms of Di-ethyl Berylliums and Di-t-butyl Berylliums

The potential energy surfaces (PES) of unimolecular dissociation reactions for di-ethyl beryllium and di-t-butyl beryllium are investigated by B3LYP, CCSD(T), and G3B3 approaches. Possible reaction pathways through either the radical or transition state (TS) of the molecules are considered. The geometries, vibrational frequencies and relative energies for various stationary points are determined. From the study of energetics, the TS pathways arising from concerted molecular eliminations are indicated to be the main dissociation pathways for both molecules. The PES differences of the dissociation reactions are investigated. The activation energies and rate constants will be helpful for investigating the predictive ability of the reaction in further theoretical and experimental research.

Mechanistic Aspects Regarding the Elimination of H2O2 from C(4a)-Hydroperoxyflavin. The Role of a Proton Shuttle Required for H2O2 Elimination

DFT calculations presented for C(4a)-hydroperoxyflavin (C(4a)-FLHOOH) at the B3LYP/6-311+G(d,p) level suggest a new mechanism for the elimination of H2O2. The calculated activation barrier for a concerted four-centered elimination (ΔE(‡) = 32.86 kcal/mol) strongly suggests that in the absence of interactions with the local environment a spontaneous elimination is not feasible. A proton shuttle from the N5 hydrogen to the proximal oxygen of the OOH moiety involving three water molecules has an activation barrier that is reduced to 17.11 kcal/mol. Calculations that utilize CH3OH to model the role of a local Thr or Ser residue shows that an alcohol functionality hydrogen bonded to the N5 H-atom can catalyze the elimination of H2O2 with a free energy of activation of 21.5 kcal/mol. Interaction of amines and amide residues (CH3NH2 and CH3(C═O)NH2) with the N5 locus of C(4a)-hydroperoxyflavin markedly reduce the activation barrier for H2O2 elimination relative to the concerted pathway. Proton transfer from a COOH group (ΔG(‡) = 8.36 kcal/mol) or the NH2 group of a positively charged Arg model (ΔG(‡) = 9.99 kcal/mol) to the proximal oxygen of the OOH moiety of C(4a)-FLHOOH in the TS for H2O2 elimination strongly enhances elimination of H2O2.

Computational Study of the Combustion and Atmospheric Decomposition of 2-Methylfuran

There is a growing interest in alkylfurans as potential biofuels. Recent work has highlighted the need for further study of the atmospheric oxidation mechanism of 2-methylfuran (2MF). This study utilizes the high level composite computational methods, G4 and CBS-QB3, to determine the bond dissociation energies for the C-H bond in 2MF and the reaction enthalpies and barrier heights of several of the known possible initiation reaction pathways. This study also investigates the possible subsequent low temperature reaction pathways following the addition of OH and then O2 onto the 2MF ring. The placement of the OH and O2 on the ring, either cis or trans to each other, dictates the viability of subsequent reactions. Of particular interest is the observation that 1,4 H-migrations that abstract the hydrogen bound to the same carbon as the OH have abnormally low barrier height. This dramatic decrease puts the reaction barrier lower than concerted eliminations and 6-membered ring Waddington-type reactions. In addition, a novel reaction type, described as a Waddington concerted elimination, is reported herein. This reaction, when viable, is generally more favorable than other reactions. The results presented here are of interest to combustion modelers and atmospheric chemists, particularly those working on aromatic hydrocarbons and systems with conjugated double bonds.

Density Functional Theory-Computed Mechanisms of Ethylene and Diethyl Ether Formation from Ethanol on γ-Al2O3(100)

Multiple potential active sites on the surface of γ-Al2O3 have led to debate about the role of Lewis and/or Brønsted acidity in reactions of ethanol, while mechanistic insights into competitive production of ethylene and diethyl ether are scarce. In this study, elementary adsorption and reaction mechanisms for ethanol dehydration and etherification are studied on the γ-Al2O3(100) surface using density functional theory calculations. The O atom of adsorbed ethanol interacts strongly with surface Al (Lewis acid) sites, while adsorption is weak on Brønsted (surface H) and surface O sites. Water, a byproduct of both ethylene and diethyl ether formation, competes with ethanol for adsorption sites. Multiple pathways for ethylene formation from ethanol are explored, and a concerted Lewis-catalyzed elimination (E2) mechanism is found to be the energetically preferred pathway, with a barrier of Ea = 37 kcal/mol at the most stable site. Diethyl ether formation mechanisms presented for the first time on γ-Al2O3 indicate that the most favorable pathways involve Lewis-catalyzed SN2 reactions (Ea = 35 kcal/mol). Additional novel mechanisms for diethyl ether decomposition to ethylene are reported. Brønsted-catalyzed mechanisms for ethylene and ether formation are not favorable on the (100) facet because of weak adsorption on Brønsted sites. These results explain multiple experimental observations, including the competition between ethylene and diethyl ether formation on alumina surfaces.

Case of the Missing Isomer: Pathways for Molecular Elimination in the Photoinduced Decomposition of 1,1-Dibromoethane

We report an experimental and computational study of the photodecomposition pathways of a prototypical gem-dihalide, 1,1-dibromoethane (1,1-EDB), in the condensed phase. Following photolysis of the matrix isolated parent compound in Ar at 5 K, photoproducts are observed corresponding to Br2 elimination (+ C2H4 or C2H2) and HBr elimination (+ vinyl bromide). The elimination products are observed in the matrix as complexes. In contrast to our recent studies of the photolysis of matrix isolated polyhalomethanes, no evidence for the iso-1,1-EDB species is found, although studies of the matrix isolated 1,1-dibromo-2,2,2-trifluoroethane analogue show that the isomer is the dominant photoproduct. These results are examined in the light of theoretical studies that have characterized in detail the 1,1-EDB potential energy surface (PES). For Br2 elimination, a pathway from the isomer on the singlet PES is found which involves a simultaneous Br2 loss with 1,2-hydrogen shift; this pathway lies lower in energy than a concerted three-center elimination from the parent 1,1-EDB. For HBr elimination, our previous theoretical studies [Kalume, A.; George, L.; Cunningham, N.; Reid, S. A. Chem. Phys. Lett. 2013, 556, 35-38] have demonstrated the existence of concerted (single-step) and sequential pathways that involve coupled proton and electron transfer, with the sequential pathway involving the isomer as an intermediate. Here, more extensive computational results argue against a simple radical abstraction pathway for this process, and we compare experimental and computational results to prior results from the photolysis of the structural isomer, 1,2-EDB. These steady-state experiments set the stage for ultrafast studies of the dynamics of this system, which will be important in unraveling the complex photodecomposition pathways operative in condensed phases.

Detection of Drug Bioactivation in Vivo: Mechanism of Nevirapine–Albumin Conjugate Formation in Patients

The non-nucleoside reverse transcriptase inhibitor nevirapine (NVP) is widely used for the treatment of human immunodeficiency virus type 1 (HIV-1), particularly in developing countries. Despite its therapeutic benefits, NVP has been associated with skin and liver injury in exposed patients. Although the mechanism of the tissue injury is not yet clear, it has been suggested that reactive metabolites of NVP may be involved. The detection of NVP mercapturate in the urine of patients undergoing standard antiretroviral chemotherapy indicates that NVP undergoes bioactivation in vivo. However, covalent binding of drug to protein in patients remains to be determined. In this study, we investigate the chemical basis of NVP protein adduct formation by using human serum albumin (HSA) and glutathione S-transferase pi (GSTP) as model proteins in vitro. In addition, HSA was isolated from serum samples of HIV-1 patients undergoing NVP therapy to measure NVP haptenation. Mass spectrometric analysis of 12-sulfoxyl-NVP-treated HSA revealed that the drug bound selectively to histidine (His146, His242, and His338) and a cysteine residue (Cys34). The reaction proceeds most likely by a concerted elimination-addition mechanism. This pathway was further confirmed by the observation of NVP-modified Cys47 in GSTP. Importantly, the same adduct (His146) was detected in HSA isolated from the blood of patients receiving NVP, providing direct evidence that NVP modifies protein in vivo, via the formation of a reactive metabolite.

Atmospheric Oxidation Mechanism of Phenol Initiated by OH Radical

The gas-phase oxidation mechanism of phenol initiated by OH radical was investigated using DFT and ab initio calculations. The initiation of the reaction is dominated by OH addition to ortho-position, forming P2, which subsequently combines with O2 at the ipso-position to form P2-1-OO adduct. A concerted HO2 elimination process from P2-1-OO was found to be much faster than the common ring closure to bicyclic intermediates. The HO2 elimination process from P2-1-OO forms 2-hydroxy-3,5-cyclohexadienone (HCH) as the main product and is also responsible for the experimental fact that the rate constants for reaction between P2 and O2 are about 2 orders of magnitude higher than those between other aromatic-OH adducts and O2. It was speculated that HCH would isomerize to catechol, which is thermodynamically more stable than HCH and was the experimentally observed main product, possibly through heterogeneous processes. Reaction of P2 with NO2 proceeded by addition to form P2-n-NO2 (n = 1, 3, 5), followed by HONO elimination from P2-1/3-NO2 to form catechol. The barriers for HONO elimination and catechol formation are below the separate reactants P2 and NO2, being consistent with the experimental observation of catechol in the absence of O2, while H2O elimination from P2-1/3-NO2 to form 2-nitrophenol (2NP) is hindered by high barriers. The most likely pathway for 2NP is the reaction of phenoxy radical and NO2.

DFT studies on the mechanisms of palladium-catalyzed intramolecular arylation of a silyl C(sp3)–H bond

Detailed mechanisms of the intramolecular C(sp3)–C(sp2) bond formation via C(sp3)–H bond activation of a trimethylsilyl (TMS) substituent catalyzed by palladium(0) complexes have been investigated with the aid of density functional theory (DFT) calculations. The results reveal that the favorable catalytic cycle includes oxidative addition, ligand substitution, concerted metalation deprotonation (CMD) and reductive elimination steps. The CMD was found to be the rate-determining step with an overall free energy barrier of 26.4 kcal mol−1. For the analogous CMe3-substituted substrate, the C(sp3)–H bond activation of the CMe3 substituent was calculated to have a high free energy barrier of 29.9 kcal mol−1. Our calculation results show that during the deprotonation process of the TMS C(sp3)–H bond, the adjacent Si atom stabilizes the charge accumulated on the C(sp3)–H carbon and facilitates the C(sp3)–H bond activation due to the ability of Si to engage in Si–C hypervalent bonding.

Dehydrogenation and demethanation of 2-methylpropane and propane in the gas-phase by the 16-electron complex [Ru(bipy)2(CO)]2+* chemically activated by the association of [Ru(bipy)2]2+ and CO

The reactions of [Ru(bipy)(2)](2+) with 2-methylpropane, propane, and propene have been investigated in the ICR cell of a mass spectrometer. In these reactions, the association of one molecule of each hydrocarbon was observed. When [Ru(bipy)(2)](2+) was ligated with CO, and the newly formed [Ru(bipy)(2)(CO)](2+) was allowed to react with 2-methylpropane and propane both dehydrogenation and demethanation were observed among association and substitution products. Density functional calculations were used to help elucidate the mechanism and the energy requirement for the dehydrogenation and demethanation reactions of 2-methylpropane mediated by [Ru(bipy)(2)(CO)](2+)*. These very interesting elimination reactions of [Ru(bipy)(2)(CO)](2+) are attributed to a hot 16-electron intermediate, [Ru(bipy)(2)(CO)](2+)*, formed upon ligation of [Ru(bipy)(2)](2+) with CO which has no efficient means of dissipating its internal energy in the low-pressure confines of the ICR cell. The reactions were concluded to occur via a concerted elimination mechanism rather than by oxidative addition/reductive elimination following the dissociation of one Ru-N bond.

Reductive Elimination of Alkylamines from Low-Valent, Alkylpalladium(II) Amido Complexes

A series of three-coordinate norbornylpalladium amido complexes ligated by bulky N-heterocyclic carbene (NHC) ligands were prepared that undergo reductive eliminations to form the alkyl-nitrogen bond of alkylamine products. The rates of reductive elimination reveal that complexes containing more-electron-donating amido groups react faster than those with less-electron-donating amido groups, and complexes containing more-sterically bulky amido groups undergo reductive elimination more slowly than complexes containing less-sterically bulky amido groups. Complexes ligated by more-electron-donating ancillary NHC ligands undergo reductive elimination faster than complexes ligated by less-electron-donating NHC ligands. In contrast to the reductive elimination of benzylamines from bisphosphine-ligated palladium amides, these reactions occur with retention of configuration at the alkyl group, indicating that these reductive eliminations proceed by a concerted pathway. The experimentally determined free energy barrier of 26 kcal/mol is close to the computed free energy barrier of 23.9 kcal/mol (363 K) for a concerted reductive elimination from the isolated, three-coordinate NHC-ligated palladium anilido complex.

Theoretical investigation into the low-temperature oxidation of ethylbenzene

The classical topic on the oxidation of alkylbenzene has been revisited via performing accurate theoretical calculations to address the salient features for the initial oxidation of ethylbenzene. Potential energy surfaces are mapped out for all possible reactions in the systems of (1-phenylethyl+O2 and 2-phenylethyl+O2). Reaction rate constants at the high-pressure limit are calculated for all possible reactions in these two systems. Direct H abstraction from 1-phenylethyl radical by oxygen molecule appears to be an important route for the formation of styrene from the oxidation of ethylbenzene. Concerted elimination of HO2 is predicted to contribute significantly the production of styrene from system of 2-phenylethyl+O2; especially at the atmospheric pressure and intermediate temperatures. Formation of the other major experimental product, benzaldehyde, is attributed to the unimolecular decomposition of C6H5CH2(O)CH3 rather than to unimolecular isomerisation of the two initial peroxy adducts. Kinetic and mechanistic data presented herein are instrumental for better understanding of the oxidative decomposition of ethylbenzene, i.e., major constituents of commonly formulated fuel surrogates.

Formation of Van Der Waals Complexes in Concerted Unimolecular Elimination Processes

Potential energy surfaces for three unimolecular elimination reactions: , , and were calculated using a variety of quantum chemical methods. It was shown that, in all the three cases, the transition state in the first step of the reaction leads to the production of the complex intermediates based on van der Waals interactions. In addition to the fact that the three complexes appear as intermediates on the potential energy surfaces, which means that they are not free entities, the entropy values of the two elimination products are far above those of the complexes due to their additional Sackur-Tetrode entropy. Moreover, the three vibrational frequencies of the H2O group in the (CH3)3COH complex and the H–Cl and H–F stretch frequencies in CH3CF3 and CH3CH2CH2Cl are quite different (see the various tables). The energy levels of the complexes were found to be below those of the corresponding decomposition products. Rate constants for the elimination processes were calculated from the potential energy surfaces using transition-state theory and were compared to available experimental data.

High-Pressure Rate Rules for Alkyl + O2 Reactions. 1. The Dissociation, Concerted Elimination, and Isomerization Channels of the Alkyl Peroxy Radical

The reactions of alkyl peroxy radicals (RO(2)) play a central role in the low-temperature oxidation of hydrocarbons. In this work, we present high-pressure rate estimation rules for the dissociation, concerted elimination, and isomerization reactions of RO(2). These rate rules are derived from a systematic investigation of sets of reactions within a given reaction class using electronic structure calculations performed at the CBS-QB3 level of theory. The rate constants for the dissociation reactions are obtained from calculated equilibrium constants and a literature review of experimental rate constants for the reverse association reactions. For the concerted elimination and isomerization channels, rate constants are calculated using canonical transition state theory. To determine if the high-pressure rate expressions from this work can directly be used in ignition models, we use the QRRK/MSC method to calculate apparent pressure and temperature dependent rate constants for representative reactions of small, medium, and large alkyl radicals with O(2). A comparison of concentration versus time profiles obtained using either the pressure dependent rate constants or the corresponding high-pressure values reveals that under most conditions relevant to combustion/ignition problems, the high-pressure rate rules can be used directly to describe the reactions of RO(2).

Detailed Kinetic Modeling of HCCI Combustion with Isopentanol

Isopentanol is an advanced biofuel that can be produced by micro-organisms through genetically engineered metabolic pathways. Compared to the more frequently studied ethanol, isopentanol's molecular structure has a longer carbon chain and includes a methyl branch. Its volumetric energy density is over 30 % higher than ethanol, and it is less hygroscopic. Some fundamental combustion properties of isopentanol in an HCCI engine have been characterized in a recent study by Yang and Dec (SAE 2010-01-2164). They found that for typical HCCI operating conditions, isopentanol lacks two-stage ignition properties, yet it has a higher HCCI reactivity than gasoline. The amount of intermediate temperature heat release (ITHR) is an important fuel property, and having sufficient ITHR is critical for HCCI operation without knock at high loads using intake-pressure boosting. Isopentanol shows considerable ITHR, and the amount of ITHR increases with boost, similar to gasoline. However, the individual effect of pressure and temperature on ITHR for isopentanol is still unclear. Also, the chemistry leading to ITHR for isopentanol in an HCCI engine needs to be explained. To answer these key questions, a detailed chemical kinetic model for isopentanol has been developed and used to perform HCCI engine simulations. The isopentanol model consists of low- and high-temperature chemistry based on reaction models for butanol isomers and isooctane (an alkane which a branched molecular structure similar to isopentanol). The model includes a new reaction step for concerted elimination of HO 2 from isopentanol, a process recently examined by da Silva and Bozzelli for ethanol (J. Phys. Chem. A, 113, 31, 2009). The isopentanol model was validated with rapid-compression-machine and shock-tube data over a wide range of temperatures, pressures and equivalence ratios (712 - 1205 K, 0.8 - 2.3 MPa, and 0.5 - 1.0, respectively). Excellent agreement between model predictions and experimental data was achieved. With regard to simulating HCCI combustion, the model reproduces the experimentally observed ITHR of isopentanol and its enhancement when simultaneously increasing pressure and decreasing temperature for a set combustion phasing. As seen in the HCCI experiments, the model shows that increasing the temperature for a fixed intake pressure promotes hot ignition, with little effect on ITHR.

Atmospheric chemistry of 2-ethyl hexanal: Photochemistry and oxidation in the presence of NO 2

The photolysis and oxidation mechanism of 2-ethyl hexanal (C 4H 9CH(C 2H 5)C(O)H—2-Ethal) have been investigated. The UV absorption cross sections were recorded over the range 200–350 nm at 298 K. The quantum yield for photolysis at 254 nm was determined to be 0.51 ± 0.09. Photolysis of 2-Ethal occurs by two primary paths: a concerted elimination path and a radical formation path, the latter being the main path. CO, heptane, 3-heptanol, and 3-heptanone were identified as the main products. The rate constant for the reaction with Cl atoms was determined using the relative rate technique with isopentane and cyclohexane as reference gases. The average rate constant obtained was k = (2.8 ± 0.9) × 10 −10 cm 3 molecule −1 s −1. The most important path in the oxidation mechanism was the acyl radical forming channel, which was corroborated by the analysis of the products formed via photolysis in the presence of NO 2. Those products were CO 2, CO, peroxyacetyl nitrate, peroxypropionyl nitrate, and ethyl nitrate.