Pyrolysis of Methyl Formate and the Reaction of Methyl Formate with H Atoms: Shock Tube Experiments and Statistical Rate Theory

Methyl formate (MF) is the smallest carboxylic ester and currently considered a promising alternative fuel. It can also serve as a model compound to study the combustion chemistry of the ester group, which is a typical structural feature in many biodiesel components. In the present work, the pyrolysis of MF was investigated behind reflected shock waves at temperatures between 1430 and 2070 K at a nominal pressure of 1.1 bar. Both time-resolved hydrogen atom resonance absorption spectroscopy (H-ARAS) and time-resolved time-of-flight mass spectrometry (TOF-MS) were used for species detection. Additionally, the reaction of MF and perdeuterated MF-d₄ with H atoms was investigated at temperatures between 1000 and 1300 K at nominal pressures of 0.4 and 1.1 bar with H-ARAS. In the latter experiments, ethyl iodide served as precursor for H atoms. Rate coefficients of seven parallel unimolecular decomposition channels of MF and five parallel reaction channels of the MF + H reaction were calculated from statistical rate theory on the basis of molecular and transition state data from quantum chemical calculations. These calculated rate coefficients were implemented into an MF pyrolysis/oxidation mechanism from the literature, and the experimental concentration-time profiles of H (from ARAS) as well as MF, CH₃OH, HCHO, and CO (from TOF-MS) were modeled. It turned out that the literature mechanism, which was originally validated against flow-reactor experiments, ignition delay times, and laminar burning velocities, was generally able to fit also the concentration-time profiles from the shock tube experiments reasonably well. The agreement could still be improved by substituting the original rate coefficients, which were estimated from structure-reactivity relationships, by the values calculated from statistical rate theory in the present work. Details of the channel branching are discussed, and the updated mechanism is given, also in machine-readable form.

The unimolecular decomposition of dimethoxymethane: channel switching as a function of temperature and pressure

Branching ratios of competing unimolecular reactions often exhibit a complicated temperature and pressure dependence that makes modeling of complex reaction systems in the gas phase difficult. In particular, the competition...

Kinetics of the Reactions of Hydroxyl Radicals with Furan and Its Alkylated Derivatives 2-Methyl Furan and 2,5-Dimethyl Furan

Furans are promising second generation biofuels with comparable energy densities to conventional fossil fuels. Combustion of furans is initiated and controlled to a large part by reactions with OH radicals, the kinetics of which are critical to understand the processes occurring under conditions relevant to low-temperature combustion. The reactions of OH radicals with furan (OH + F, R1), 2-methyl furan (OH + 2-MF, R2), and 2,5-dimethyl furan (OH + 2,5-DMF, R3) have been studied in this work over the temperature range 294-668 K at pressures between 5 mbar and 10 bar using laser flash photolysis coupled with laser-induced fluorescence (LIF) spectroscopy to generate and monitor OH radicals under pseudo-first-order conditions. Measurements at p ≤ 200 mbar were made in N₂, using H₂O₂ or (CH₃)₃COOH radical precursors, while those at p ≥ 2 bar were made in He, using HNO₃ as the radical precursor. The kinetics of reactions R1-R3 were observed to display a negative dependence on temperature over the range investigated, indicating the dominance of addition reactions under such conditions, with no significant dependence on pressure observed. Master equation calculations are in good agreement with the observed kinetics, and a combined parametrization of addition channels and abstraction channels for R1-R3 is provided on the basis of this work and previous shock tube measurements at higher temperatures. This work significantly extends the temperature range previously investigated for R1 and represents the first temperature-dependent measurements of R2 and R3 at temperatures relevant for atmospheric chemistry and low-temperature combustion.

On the influence of water on urea condensation reactions: a theoretical study

The influence of water molecules on the kinetics of urea condensation reactions was studied with high-level quantum chemical methods and statistical rate theory. The study focuses on the production of biuret, triuret, and cyanuric acid from urea because of their relevance as unwanted byproducts in the urea-based selective catalytic reduction (urea-SCR) exhaust after treatment of Diesel engines. In order to characterize the potential energy surfaces and molecular reaction pathways, calculations with explicitly-correlated coupled-cluster methods were performed. It turned out that the reactions proceed via pre-reactive complexes and the inclusion of one or two water molecules into the condensation mechanisms leads to a decrease of the energy barriers. This effect is particularly pronounced in the production of biuret. Due to the pre-reactive equilibria, the rates of the overall reactions can increase or decrease by incorporating water into the mechanism, depending on the temperature and water concentration. Under the conditions of urea-SCR, the studied reactions are too slow to contribute to the observed byproduct formation.

Temperature- and pressure-dependent kinetics of the competing C–O bond fission reactions of dimethoxymethane

Under typical shock tube conditions, dimethoxymethane decomposes mainly to give CH₃ + OCH₂OCH₃.

Ultrafast stimulated emission of nitrophenolates in organic and aqueous solutions

Early-time dynamics of nitroaromatics and its coressponding bases can give valuable insights into photo-induced reactions relevant to atmospheric and environmental processes. In this work, femtosecond broadband absorption spectroscopy between 350 and 700 nm has been applied to explore the ultrafast dynamics of o-, p- and m-nitrophenol anions (NP-) in basic organic and aqueous solution. Excitation at 400 nm promotes these compounds into the first bright electronic singlet state, which is a charge-transfer state. A surprising finding for all nitrophenolates was a characteristic, spectrally broad stimulated emission (SE) from the electronically excited state into the ground state. The corresponding lifetime was on the order of a few hundred femtoseconds for o- and p-NP- while it was roughly ten times larger for m-NP-. In line with earlier observations, the SE is governed by an out-of-plane torsional motion of the nitro group, leading to a close energetic approach of the relevant electronically excited singlet and ground states. Subsequent dynamics can be assigned to excited state absorption and ground state relaxation due to energy dissipation of the vibrational modes to the solvent that occur for up to several tens of picoseconds. No longer-lasting transient absorption (TA) was found; instead, a complete recovery of the ground state bleaching was observed indicating that triplet state relaxation is either not significantly involved in this spectral part or shifted to other regions. In the aqueous system, time constants for all processes are much smaller than in organic solution, a fact that can be explained by the larger dipole moment of the solvent and the correspondingly stronger intermolecular coupling between NP- and the aqueous solvent.

Kinetics in the real world: linking molecules, processes, and systems

Unravelling elementary steps, reaction pathways, and kinetic mechanisms is key to understanding the behaviour of many real-world chemical systems that span from the troposphere or even interstellar media to engines and process reactors. Recent work in chemical kinetics provides detailed information on the reactive changes occurring in chemical systems, often on the atomic or molecular scale. The optimisation of practical processes, for instance in combustion, catalysis, battery technology, polymerisation, and nanoparticle production, can profit from a sound knowledge of the underlying fundamental chemical kinetics. Reaction mechanisms can combine information gained from theory and experiments to enable the predictive simulation and optimisation of the crucial process variables and influences on the system's behaviour that may be exploited for both monitoring and control. Chemical kinetics, as one of the pillars of Physical Chemistry, thus contributes importantly to understanding and describing natural environments and technical processes and is becoming increasingly relevant for interactions in and with the real world.

endo-Cyclization of unsaturated RO2 radicals from the gas-phase ozonolysis of cyclohexadienes

Unsaturated RO₂ radicals from the ozonolysis of cyclodienes can readily undergo an endo-cyclization step under atmospheric conditions forming a new ring-containing RO₂ radical after further O₂ addition. This path represents an extension of the atmospheric autoxidation scheme forming highly oxidized multifunctional organic compounds (HOMs). HOMs play an important role for Earth's organic aerosol burden.

Exploring the chemical kinetics of partially oxidized intermediates by combining experiments, theory, and kinetic modeling

Partially oxidized intermediates play a central role in combustion and atmospheric chemistry. In this perspective, we focus on the chemical kinetics of alkoxy radicals, peroxy radicals, and Criegee intermediates, which are key species in both combustion and atmospheric environments. These reactive intermediates feature a broad spectrum of chemical diversity. Their reactivity is central to our understanding of how volatile organic compounds are degraded in the atmosphere and converted into secondary organic aerosol. Moreover, they sensitively determine ignition timing in internal combustion engines. The intention of this perspective article is to provide the reader with information about the general mechanisms of reactions initiated by addition of atomic and molecular oxygen to alkyl radicals and ozone to alkenes. We will focus on critical branching points in the subsequent reaction mechanisms and discuss them from a consistent point of view. As a first example of our integrated approach, we will show how experiment, theory, and kinetic modeling have been successfully combined in the first infrared detection of Criegee intermediates during the gas phase ozonolysis. As a second example, we will examine the ignition timing of n-heptane/air mixtures at low and intermediate temperatures. Here, we present a reduced, fuel size independent kinetic model of the complex chemistry initiated by peroxy radicals that has been successfully applied to simulate standard n-heptane combustion experiments.

Low-Temperature Autoignition of Diethyl Ether/O2 Mixtures: Mechanistic Considerations and Kinetic Modeling

Autoignition processes are of fundamental kinetic importance as well as of practical relevance for combustion devices. In recent years, diethyl ether (DEE) has attracted increasing attention as a diesel additive and also serves as a test compound in fire-safety-related studies. In the present work, a kinetically parameterized reaction mechanism for the autoignition of DEE is developed. It consists of a DEE-specific part supplemented by a base mechanism taken from the literature that contains the C1/C2 hydrocarbon and the H2/O2 reaction systems. The complete mechanism is validated against experimental ignition delay times available from the literature for temperatures ranging from 500 to 1300 K and reactant pressures between 3 and 5 bar (T=500−900 K) and between 10 and 40 bar (T=900−1300 K). The absolute values and the temperature dependence of the ignition delay times are satisfactorily reproduced. This includes important autoignition characteristics such as one- and two-stage ignitions and the so-called negative temperature coefficient regime where ignition delay times increase with temperature. Detailed kinetic-mechanistic explanations for all these phenomena are given.

Collisional Relaxation of NCN: An Experimental Study with Laser-Induced Fluorescence

Collisional relaxation of NCN produced by photolysis of NCN3 at a wavelength of 248 nm has been investigated with laser-induced fluorescence (LIF) in a temperature range of 240–293 K and a pressure range of 10–800 mbar with different bath gases (He, Ne, Ar, Kr, H2, N2, O2, and N2O). LIF excitation spectra were recorded, and LIF intensity-time profiles experimentally obtained at an excitation wavelength of 329.01 nm have been fitted with biexponential functions. The pressure and bath gas dependence of the rate coefficients obtained was rationalized in terms of a simple Landau–Teller/Schwartz–Slawsky–Herzfeld model, and notable influences of vibrational energy transfer to the rate of the overall relaxation process have been found. Evidence was obtained for a two-channel vibrational relaxation in NCN.

Ultrafast Dynamics of o-Nitrophenol: An Experimental and Theoretical Study

The photolysis of o-nitrophenol (o-NP), a typical push-pull molecule, is of current interest in atmospheric chemistry as a possible source of nitrous acid (HONO). To characterize the largely unknown photolysis mechanism, the dynamics of the lowest lying excited singlet state (S1) of o-NP was investigated by means of femtosecond transient absorption spectroscopy in solution, time-resolved photoelectron spectroscopy (TRPES) in the gas phase and quantum chemical calculations. Evidence of the unstable aci-nitro isomer is provided both in the liquid and in the gas phase. Our results indicate that the S1 state displays strong charge transfer character, which triggers excited state proton transfer from the OH to the NO2 group as evidenced by a temporal shift of 20 fs of the onset of the photoelectron spectrum. The proton transfer itself is found to be coupled to an out-of-plane rotation of the newly formed HONO group, finally leading to a conical intersection between S1 and the ground state S0. In solution, return to S0 within 0.2-0.3 ps was monitored by stimulated emission. As a competitive relaxation channel, ultrafast intersystem crossing to the upper triplet manifold on a subpicosecond time scale occurs both in solution and in the gas phase. Due to the ultrafast singlet dynamics, we conclude that the much discussed HONO split-off is likely to take place in the triplet manifold.

Thermal Decomposition of NCN: Shock-Tube Study, Quantum Chemical Calculations, and Master-Equation Modeling

The thermal decomposition of cyanonitrene, NCN, was studied behind reflected shock waves in the temperature range 1790-2960 K at pressures near 1 and 4 bar. Highly diluted mixtures of NCN3 in argon were shock-heated to produce NCN, and concentration-time profiles of C atoms as reaction product were monitored with atomic resonance absorption spectroscopy at 156.1 nm. Calibration was performed with methane pyrolysis experiments. Rate coefficients for the reaction (3)NCN + M → (3)C + N2 + M (R1) were determined from the initial slopes of the C atom concentration-time profiles. Reaction R1 was found to be in the low-pressure regime at the conditions of the experiments. The temperature dependence of the bimolecular rate coefficient can be expressed with the following Arrhenius equation: k1(bim) = (4.2 ± 2.1) × 10(14) exp[-242.3 kJ mol(-1)/(RT)] cm(3) mol(-1) s(-1). The rate coefficients were analyzed by using a master equation with specific rate coefficients from RRKM theory. The necessary molecular data and energies were calculated with quantum chemical methods up to the CCSD(T)/CBS//CCSD/cc-pVTZ level of theory. From the topography of the potential energy surface, it follows that reaction R1 proceeds via isomerization of NCN to CNN and subsequent C-N bond fission along a collinear reaction coordinate without a tight transition state. The calculations reproduce the magnitude and temperature dependence of the rate coefficient and confirm that reaction R1 is in the low-pressure regime under our experimental conditions.

Kinetics of the unimolecular reaction of CH2OO and the bimolecular reactions with the water monomer, acetaldehyde and acetone under atmospheric conditions

The rate coefficients of the unimolecular reaction of CH₂OO and the bimolecular reactions with the water monomer and carbonyls were measured.

The reactions of N-methylformamide and N,N-dimethylformamide with OH and their photo-oxidation under atmospheric conditions: experimental and theoretical studies

The atmospheric oxidation of amides is studied with a combination of laser photolysis and smog chamber experiments along with quantum chemical and statistical rate theory calculations.

Kinetics of the Reactions of Hydroxyl Radicals with Diacetylene and Vinylacetylene

Highly unsaturated hydrocarbons like diacetylene (C4H2) or vinylacetylene (C4H4) are important intermediates in combustion that can have impact on soot formation. One of their major loss channels is reaction with hydroxyl radicals (OH). We studied the reactions C4H2 + OH → products (1) and C4H4 + OH → products (2) in a quasi-static reactor with helium as bath gas. The hydroxyl radicals were produced by laser flash-photolysis of nitric acid at a wavelength of 248 nm and detected by laser-induced fluorescence with excitation at 282 nm. The rate coefficients were obtained from the intensity-time profiles under pseudo-first order conditions with respect to OH. We found a virtually temperature-independent rate coefficient for reaction (1): k 1 = (1.0 ± 0.3) × 10−11 cm3 s –1 (T = 290–670 K, P = 2.7–30.5 bar) and a weakly negative temperature-dependent rate coefficient for reaction (2): k 2(T) = (6.4 ± 1.9) × 10−12 exp (486 K/T) cm3 s –1 (T = 295–740 K, P = 1.7–19.2 bar). For neither of the two reactions pressure dependence was observed. From comparisons with analogous reaction systems, we conclude that the dominating reaction pathway is OH addition, where in the case of C4H4 the double bond is preferred over the triple bond.

Pyrolysis of Ethyl Iodide as Hydrogen Atom Source: Kinetics and Mechanism in the Temperature Range 950–1200 K

Ethyl iodide is a well known H atom precursor in shock tube experiments. In the present work, we study peculiarities, when C2H5I is used under conditions, where its decomposition is not longer fast compared to consecutive bimolecular reactions. On the basis of shock tube experiments with detection of H and I atoms by resonance absorption spectrometry, accompanied by quantum chemical (CCSD(T)/6-311G//CCSD/6-311G) and statistical rate theory calculations, we propose a small mechanism (5 reactions, 7 species) and kinetic data, which allow an adequate description of C2H5I pyrolysis as a H atom source down to temperatures between 950 and 1200 K at pressures ranging from 1 to 4 bar: C2H5I→C2H5 + I (1), k 1 = 9.9 × 1012 exp(−23200 K/T) s−1; C2H5 + M→C 2H4 + H + M (2), k 2 = 1.7 × 10−6 exp(−16800 KT) cm3 s−1 [D. L. Baulch et al., J. Phys. Chem. Ref. Data 34 (2005) 757]; C2H5I→C2H4 + HI (3), k 3 = 1.7 × 1013 exp(−26680 KT) s−1; H + HI→H2 + I (4), k 4 = 7.9 × 10−11 exp(−330 KT) cm3 s−1 [D. L. Baulch et al., J. Phys. Chem. Ref. Data 10(Suppl. 1) (1981) 1]; C2H5I + H→C2H5 + HI (5), k 5 = 7.0 × 10−9 exp(−3940 KT) cm3 s−1. The latter bimolecular abstraction step turned out crucial for an adaquate d escription of the hydrogen atom concentration-time profiles in the above mentioned temperature and pressure range for initial concentrations [C2H5I]0 > 2 × 1013 cm−3 corresponding to mole fractions > 1 ppm.

Experimental and theoretical investigation of the kinetics of the reaction of atomic chlorine with 1,4-dioxane

The rate coefficients for the reaction of 1,4-dioxane with atomic chlorine were measured from T = 292-360 K using the relative rate method. The reference reactant was isobutane and the experiments were made in argon with atomic chlorine produced by photolysis of small concentrations of Cl2. The rate coefficients were put on an absolute basis by using the published temperature dependence of the absolute rate coefficients for the reference reaction. The rate coefficients for the reaction of Cl with 1,4-dioxane were found to be independent of total pressure from p = 290 to 782 Torr. The experimentally measured rate coefficients showed a weak temperature dependence, given by k(exp)(T) = (8.4(-2.3)(+3.1)) × 10(-10) exp(-(470 ± 110)/(T/K)) cm3 molecule (-1) s(-1). The experimental results are rationalized in terms of statistical rate theory on the basis of molecular data obtained from quantum-chemical calculations. Molecular geometries and frequencies were obtained from MP2/aug-cc-pVDZ calculations, while single-point energies of the stationary points were computed at CCSD(T) level of theory. The calculations indicate that the reaction proceeds by an overall exothermic addition-elimination mechanism via two intermediates, where the rate-determining step is the initial barrier-less association reaction between the chlorine atom and the chair conformer of 1,4-dioxane. This is in contrast to the Br plus 1,4-dioxane reaction studied earlier, where the rate-determining step is a chair-to-boat conformational change of the bromine-dioxane adduct, which is necessary for this reaction to proceed. The remarkable difference in the kinetic behavior of the reactions of 1,4-dioxane with these two halogen atoms can be consistently explained by this change in the reaction mechanism.