Oxidation Kinetics and Thermodynamics of Resonance-Stabilized Radicals: The Pent-1-en-3-yl + O2 Reaction

Gas-Phase Oxidation of Atmospherically Relevant Unsaturated Hydrocarbons by Acyl Peroxy Radicals

This study assesses the atmospheric impact of reactions between unsaturated hydrocarbons such as isoprene and monoterpenes and peroxy radicals containing various functional groups. We find that reactions between alkenes and acyl peroxy radicals have reaction rates high enough to be feasible in the atmosphere and lead to high molar mass accretion products. Moreover, the reaction between unsaturated hydrocarbons and acyl peroxy radicals leads to an alkyl radical, to which molecular oxygen rapidly adds. This finding is confirmed by both theoretical calculations and experiments. The formed perester peroxy radical may either undergo further H-shift reactions or react bimolecularly. The multifunctional oxygenated compounds formed through acyl peroxy radical + alkene reactions are potentially important contributors to particle formation and growth. Thus, acyl peroxy radical-initiated oxidation chemistry may need to be included in atmospheric models.

Investigating the Association Reactions of HOCH2CO and HOCHCHO with O2: A Quantum Computational and Master Equation Study

Glycolaldehyde, HOCH₂CHO, is an important multifunctional atmospheric trace gas formed in the oxidation of ethylene and isoprene and emitted directly from burning biomass. The initial step in the atmospheric photooxidation of HOCH₂CHO yields HOCH₂CO and HOCHCHO radicals; both of these radicals react rapidly with O₂ in the troposphere. This study presents a comprehensive theoretical investigation of the HOCH₂CO + O₂ and HOCHCHO + O₂ reactions using high-level quantum chemical calculations and energy-grained master equation simulations. The HOCH₂CO + O₂ reaction results in the formation of a HOCH₂C(O)O₂ radical, while the HOCHCHO + O₂ reaction yields (HCO)₂ + HO₂. Density functional theory calculations have identified two open unimolecular pathways associated with the HOCH₂C(O)O₂ radical that yield HCOCOOH + OH or HCHO + CO₂ + OH products; the former novel bimolecular product pathway has not been previously reported in the literature. Master equation simulations based on the potential energy surface calculated here for the HOCH₂CO + O₂ recombination reaction support experimental product yield data from the literature and indicate that, even at total pressures of 1 atm, the HOCH₂CO + O₂ reaction yields ∼11% OH at 298 K.

An experimental and computational study of the reaction between 2-methylallyl radicals and oxygen molecules: optimizing master equation parameters with trace fitting

We have investigated the reaction between 2-methylallyl radicals and oxygen molecules with experimental and computational methods. Kinetic experiments were conducted in a tubular laminar flow reactor using laser photolysis for radical production and photoionization mass spectrometry for detection. The reaction was investigated as a function of temperature (203-730 K) and pressure (0.2-9 torr) in helium and nitrogen bath gases. At low temperatures (T < 410 K), the reaction proceeds by a barrierless reaction to form 2-methylallylperoxyl. Equilibration of the peroxyl adduct and the reactants was observed between 350-410 K. Measurements were extended to even higher temperatures, up to 730 K, but no reaction could be observed. Master equation simulations of the reaction system were performed with the MESMER program. Kinetic parameters in the master equation model were optimized by direct fitting to time-resolved experimental 2-methylallyl traces. Trace fitting is a recently implemented novel feature in MESMER. The trace approach was compared with the more traditional approach where one uses experimental rate coefficients for parameter optimization. The optimized parameters yielded by the two approaches are very similar and do an excellent job at reproducing the experimental data. The optimized master equation model was then used to simulate the reaction under study over a wide temperature and pressure range, from 200 K and 0.01 bar to 1500 K and 100 bar. The simulations predict a small phenomenological rate coefficient under autoignition conditions; about 1 × 10^-18 cm³ s^-1 at 400 K and 5 × 10^-16 cm³ s^-1 at 1000 K. We provide modified Arrhenius expressions in PLOG format for the most important product channels to facilitate the use of our results in combustion models.

An Experimental and Computational Study of the Reaction between Pent-3-en-2-yl Radicals and Oxygen Molecules: Switching from Pure Stabilisation to Pure Decomposition with Increasing Temperature

We have used laser-photolysis – photoionization mass spectrometry, quantum chemical calculations, and master equation simulations to investigate the kinetics of the reaction between (E/Z)-pent-3-en-2-yl (CH3CHCHCHCH3), a resonance-stabilised hydrocarbon radical, and...

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.

Heteroatom (N, P, As, Sb, Bi) Effects on the Resonance-Stabilized 2-, 3-, and 4-Picolyl Radicals

Important recent experimental studies have allowed the isomer-selective identification of the 2-, 3-, and 4-picolyl radicals. The picolyl radicals and their valence isoelectronic P, As, Sb, and Bi congeners are investigated here. For the three observed parent radicals, the theoretical ionization potentials agree with experiment to within 0.02 eV. Two rules are proposed for predicting vertical ionization potentials (E_VIE) and relative energies. The E_VIE values for these radicals will be higher when large percentages of the SOMO orbitals are distributed on the atoms with greater electronegativities. The cations of these systems were also studied along with the closed-shell methylpyridines and their P, As, Sb, and Bi analogs. The energies for the cationic species will lie lower when high percentages of π natural localized molecular orbitals occur on the more electronegative atoms. The structures of the 2- and 4-isomers strongly depend upon the heteroatoms, with the C-C linkages adopting a single-double alternating bond manner when the heteroatoms become heavier. The 3-isomers adopt roughly equal C-C bond distances with small changes from N to Bi.

Atomic Partial Charges as Descriptors for Barrier Heights

Atomic partial charges are found to be valuable descriptors for barrier heights of unimolecular reactions due to the considerable information about the electronic structure embedded in them. If the chemical changes of the reactions are somewhat centralized at a single atom, the respective partial charge is a potentially meaningful descriptor and might outperform bond dissociation energies as descriptors. We propose that atomic partial charges should be considered as barrier height descriptors in future research.