Theoretical Investigation of Key Properties of the Pyrolysis of Methyl, Ethyl, and Dimethyl Dioxolane Isomers.

Cyclic acetals are considered as carbon-neutral fuels that can be produced from biomass and renewable electricity. Recent investigations on 1,3-dioxolane, a five-membered cyclic acetal, revealed that unimolecular decomposition through H-atom migration within the ring governs thermal decomposition. For methyl- and ethyl-substituted dioxolane compounds, very limited information on thermodynamic, transport, bond dissociation, and thermal decomposition properties is available. The present study remedies this lack of information by providing these properties for methyl-, ethyl-, and dimethyl-substituted dioxolanes in a systematic manner. While adding substituents to the dioxolane ring has only a minor effect on the bond dissociation energies, the barrier heights for H-atom migration are clearly affected by the position of the substituents. Notably, the corresponding transition states are preferably in the boat ring configuration. However, two of the substituted dioxolanes do not allow for this configuration because of their respective bonding structures, resulting in larger barrier heights. The properties provided here will aid the detailed chemical kinetic modeling of substituted dioxolane combustion chemistry.

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.

Reaction kinetics of hydrogen atom abstraction from isopentanol by the H atom and HO2˙ radical.

Isopentanol is a potential next-generation biofuel for future applications to Homogeneous Charge Compression Ignition (HCCI) engine concepts. To provide insights into the combustion behavior of isopentanol, especially to its auto-ignition behavior which is linked both to efficiency and pollutant formation in real combustion systems, detailed quantum chemical studies for crucial reactions are desired. H-Abstraction reaction rates from fuel molecules are key initiation steps for chain branching required for auto-ignition. In this study, rate constants are determined for the hydrogen atom abstraction reactions from isopentanol by the H atom and HO2˙ radical by implementing the CBS-QB3 composite method. For the treatment of the internal rotors, a Pitzer-Gwinn-like approximation is applied. On comparing the computed reaction energies, the highest exothermicity (ΔE = -46 kJ mol-1) is depicted for Hα abstraction by the H atom whereas the lowest endothermicity (ΔE = 29 kJ mol-1) is shown for the abstraction of Hα by the HO2˙ radical. The formation of hydrogen bonding is found to affect the kinetics of the H atom abstraction reactions by the HO2˙ radical. Further above 750 K, the calculated high pressure limit rate constants indicate that the total contribution from delta carbon sites (Cδ) is predominant for hydrogen atom abstraction by the H atom and HO2˙ radical.

Theoretical Investigation of Intramolecular Hydrogen Shift Reactions in 3-Methyltetrahydrofuran (3-MTHF) Oxidation.

3-Methyltetrahydrofuran (3-MTHF) is proposed to be a promising fuel component among the cyclic oxygenated species. To have detailed insight of its combustion kinetics, intramolecular hydrogen shift reactions for the ROO to QOOH reaction class are studied for eight ROO isomers of 3-MTHF. Rate constants of all possible reaction paths that involve formation of cyclic transition states are computed by employing the CBS-QB3 composite method. A Pitzer-Gwinn-like approximation has been applied for the internal rotations in reactants, products, and transition states for the accurate treatment of hindered rotors. Calculated relative barrier heights highlight that the most favorable reaction channel proceeds via a six membered transition state, which is consistent with the computed rate constants. Comparing total rate constants in ROO isomers of 3-MTHF with the corresponding isomers of methylcyclopentane depicts faster kinetics in 3-MTHF than methylcyclopentane reflecting the effect of ring oxygen on the intramolecular hydrogen shift reactions.