Singlet and triplet potential surfaces for the O2+C2H4 reaction

Reaction of Singlet Oxygen with the Ethylene Group: Implications for Electrolyte Stability in Li-Ion and Li-O2 Batteries

Recent experimental and computational evidence indicates that singlet oxygen (¹O₂) attacks the ethylene group (-CH₂-CH₂-) in ethylene carbonate (EC) leading to degradation in Li-ion batteries employing EC as the electrolyte solvent [J. Phys. Chem. A 2018, 122, 8828-8839]. Here, we employ computational quantum chemistry to explore this mechanism in detail for a large set of organic molecules. Benchmark calculations comparing density functional theory to the complete active space second-order perturbation theory and internally contracted multireference configuration interaction indicate that the M11 functional adequately captures trends in the transition-state energies for this mechanism. Based on our results, we recommend that solvents which include the ethylene group should be avoided in Li-ion and Li-O₂ batteries where ¹O₂ is generated unless neighboring functional groups raise the reaction barrier to avoid this decomposition pathway.

The reaction of O(3P) with alkynes: a dynamic and computational study focusing on formyl radical production

Production of formyl radical, HCO, from reactions of O(³P) with alkynes (acetylene, propyne, 1-butyne, and 1-pentyne) has been investigated using cavity ringdown laser absorption spectroscopy (CRDLAS) and computational methods. No HCO was detected from reaction with acetylene, while the amount of HCO increased for propyne and 1-butyne, dropping off somewhat for 1-pentyne. These results differ from trends previously observed for reactions of O(³P) with alkenes, which exhibit the largest HCO production for the smallest alkene and drop off as the alkene size increases. Computational studies employing density functional and coupled cluster methods have been employed to investigate the triplet and singlet state pathways for HCO production. Because intersystem crossing (ISC) has been shown to be important in these processes, the minimum energy crossing point (MECP) between the triplet and singlet surfaces has been studied. We find the MECP for propyne to possess C₁ symmetry and to lie lower in energy than previous studies have found. Natural Bond Orbital and Natural Resonance Theory analyses have been performed to investigate the changes in spin density and bond order along the reaction pathways for formation of HCO. Explanations are suggested for the trend in HCO formation observed for the alkynes. The trend in alkyne HCO yield also is compared and contrasted with the trend previously observed for the alkenes.

A fully diastereoselective oxidation of a mesoionic dipole with triplet molecular oxygen

Oxidations with molecular oxygen are ubiquitous processes in biological systems where cofactor-dependent enzymes activate either oxygen or hydrogen peroxide to induce multichannel pathways. In stark contrast, such slow atmospheric oxidations are seldom harnessed in chemical synthesis and analysis. The present study unveils an unusual aerobic oxidation of a mesoionic dipole leading easily to a more functionalized skeleton. Although the synthetic scope has not been explored, two key considerations emerge from this transformation, as it proceeds with complete diastereoselection and could be successfully extrapolated to structurally related mesoionic chirons without racemization. How this oxidation actually occurs proved to be puzzling from the onset and only high-level computation reveals a cascade transformation, whose results reconcile theory and experiment. Hopefully, the mechanistic insights should help us to understand better the autoxidative reactions of organic molecules.

Direct Dynamics Simulation of the Thermal 3CH2 + 3O2 Reaction. Rate Constant and Product Branching Ratios

The reaction of ³CH₂ with ³O₂ is of fundamental importance in combustion, and the reaction is complex as a result of multiple extremely exothermic product channels. In the present study, direct dynamics simulations were performed to study the reaction on both the singlet and triplet potential energy surfaces (PESs). The simulations were performed at the UM06/6-311++G(d,p) level of theory. Trajectories were calculated at a temperature of 300 K, and all reactive trajectories proceeded through the carbonyl oxide Criegee intermediate, CH₂OO, on both the singlet and triplet PESs. The triplet surface leads to only one product channel, H₂CO + O(³P), while the singlet surface leads to eight product channels with their relative importance as CO + H₂O > CO + OH + H ∼ H₂CO + O(¹D) > HCO + OH ∼ CO₂ + H₂ ∼ CO + H₂ + O(¹D) > CO₂ + H + H > HCO + O(¹D) + H. The reaction on the singlet PES is barrierless, consistent with experiment, and the total rate constant on the singlet surface is (0.93 ± 0.22) × 10^-12 cm³ molecule^-1 s^-1 in comparison to the recommended experimental rate constant of 3.3 × 10^-12 cm³ molecule^-1 s^-1. The simulation product yields for the singlet PES are compared with experiment, and the most significant differences are for H, CO₂, and H₂O. The reaction on the triplet surface is also barrierless, inconsistent with experiment. A discussion is given of the need for future calculations to address (1) the barrier on the triplet PES for ³CH₂ + ³O₂ → ³CH₂OO, (2) the temperature dependence of the ³CH₂ + ³O₂ reaction rate constant and product branching ratios, and (3) the possible non-RRKM dynamics of the ¹CH₂OO Criegee intermediate.

Ab initio and transition state theory study of the OH + HO2 → H2O + O2(3Σg-)/O2(1Δg) reactions: yield and role of O2(1Δg) in H2O2 decomposition and in combustion of H2

Reactions of hydroxyl (OH) and hydroperoxyl (HO₂) are important for governing the reactivity of combustion systems. We performed post-CCSD(T) ab initio calculations at the W3X-L//CCSD = FC/cc-pVTZ level to explore the triplet ground-state and singlet excited-state potential energy surfaces of the OH + HO₂ → H₂O + O₂(³Σ_g^-)/O₂(¹Δ_g) reactions. Using microcanonical and multistructural canonical transition state theories, we calculated the rate constant for the triplet and singlet channels over the temperature range 200-2500 K, represented by k(T) = 3.08 × 10¹²T^0.07 exp(1151/RT) + 8.00 × 10¹²T^0.32 exp(-6896/RT) and k(T) = 2.14 × 10⁶T^1.65 exp(-2180/RT) in cm³ mol^-1 s^-1, respectively. The branching ratios show that the yield of singlet excited oxygen is small (<0.5% below 1000 K). To ascertain the importance of singlet oxygen channel, our new kinetic information was implemented into the kinetic model for hydrogen combustion recently updated by Konnov (Combust. Flame, 2015, 162, 3755-3772). The updated kinetic model was used to perform H₂O₂ thermal decomposition simulations for comparison against shock tube experiments performed by Hong et al. (Proc. Combust. Inst., 2013, 34, 565-571), and to estimate flame speeds and ignition delay times in H₂ mixtures. The simulation predicted a larger amount of O₂(¹Δ_g) in H₂O₂ decomposition than that predicted by Konnov's original model. These differences in the O₂(¹Δ_g) yield are due to the use of a higher ab initio level and a more sophisticated methodology to compute the rate constant than those used in previous studies, thereby predicting a significantly larger rate constant. No effect was observed on the rate of the H₂O₂ decomposition and on the flame speeds and ignition delay times of different H₂-oxidizer mixtures. However, if the oxidizer is seeded with O₃, small differences appear in the flame speed. Given that O₂(¹Δ_g) is much more reactive than O₂(³Σ_g^-), we do not preclude an effect of the singlet channel of the titled reaction in other combustion systems, especially in systems where excited oxygen plays an important role.

Temperature and pressure dependent rate coefficients for the reaction of C2H4 + HO2 on the C2H4O2H potential energy surface

The potential energy surface (PES) for reaction C2H4 + HO2 was examined by using the quantum chemical methods. All rates were determined computationally using the CBS-QB3 composite method combined with conventional transition state theory(TST), variational transition-state theory (VTST) and Rice-Ramsberger-Kassel-Marcus/master-equation (RRKM/ME) theory. The geometries optimization and the vibrational frequency analysis of reactants, transition states, and products were performed at the B3LYP/CBSB7 level. The composite CBS-QB3 method was applied for energy calculations. The major product channel of reaction C2H4 + HO2 is the formation C2H4O2H via an OH(···)π complex with 3.7 kcal/mol binding energy which exhibits negative-temperature dependence. We further investigated the reactions related to this complex, which were ignored in previous studies. Thermochemical properties of the species involved in the reactions were determined using the CBS-QB3 method, and enthalpies of formation of species were compared with literature values. The calculated rate constants are in good agreement with those available from literature and given in modified Arrhenius equation form, which are serviceable in combustion modeling of hydrocarbons. Finally, in order to illustrate the effect for low-temperature ignition of our new rate constants, we have implemented them into the existing mechanisms, which can predict ethylene ignition in a shock tube with better performance.

Finding Minimum Structures on the Seam of Crossing in Reactions of Type A + B → X: Exploration of Nonadiabatic Ignition Pathways of Unsaturated Hydrocarbons

A new theoretical approach is proposed for finding automatically minimum structures on the seam of crossing (MSX) in reactions of type A + B → X, where the artificial-force-induced reaction (AFIR) method is combined with the seam model function (SMF) approach. Its application to reactions between triplet dioxygen and unsaturated hydrocarbons provided many MSX structures. In addition to known ignition pathways, we discovered a pathway through a new type of MSX in the reaction of dioxygen with aromatic hydrocarbons; for benzene, this new pathway requires a lower energy than those of three known ignition pathways and is likely to be the most important. This demonstrates that the AFIR-SMF approach has the ability to discover unknown/unexpected MSX structures without prejudice for presumed pathways or mechanisms.