Thermochemistry of Hydroxyl and Hydroperoxide Substituted Furan, Methylfuran, and Methoxyfuran

Investigation of the Electronic and Vibrational Structures of the 2-Furanyloxy Radical Using Photoelectron Imaging and Photodetachment Spectroscopy via the Dipole-Bound State of the 2-Furanyloxide Anion

The 2-furanyloxy radical is an important chemical reaction intermediate in the combustion of biofuels and aromatic compounds. We report an investigation of its electronic and vibrational structures using photoelectron and photodetachment spectroscopy and resonant photoelectron imaging (PEI) of cryogenically cooled 2-furanyloxide anion. The electron affinity of 2-furanyloxy is measured to be 1.7573(8) eV. Two excited electronic states are observed at excitation energies of 2.14 and 2.82 eV above the ground state. Photodetachment spectroscopy reveals a dipole-bound state 0.0143 eV below the detachment threshold and 25 vibrational Feshbach resonances for the 2-furanyloxide anion. The combination of photodetachment spectroscopy and resonant PEI yields frequencies for 18 out of a total of 21 vibrational modes for the 2-furanyloxy radical, including all six of its bending modes. The rich electronic and vibrational information will be valuable for further understanding the role of 2-furanyloxy as a key reaction intermediate of combustion and atmospheric interests.

Mechanisms and kinetics of the low-temperature oxidation of 2-methylfuran: insight from DFT calculations and kinetic simulations

The low-temperature oxidation (LTO) mechanisms of the 2-methylfuran (2-MF) biofuel and the corresponding thermodynamic and kinetic properties have been explored by density functional theory (DFT) and composite G4 methodologies as well as kinetic simulations. The O₂ addition to the main furylCH₂ radical from the methyl dehydrogenation in 2-MF forms three peroxide radicals PO1, PO2, and PO3 with the energy barriers of 15.1, 19.3, and 20.6 kcal mol^-1 and the reaction ΔG of -8.2, 5.7, and -0.1 kcal mol^-1 (298 K and 1 atm), respectively. Through hydrogen transfer followed by dehydroxylation, these nascent products evolve into stable aldehydes and cyclic ketones, which may further decompose into smaller species under the action of OH. Calculations and simulations show that the product P1 from the dehydroxylation of PO1 has a dominant population (higher than 96%) among the final products, although the temperature and pressure may influence the species profiles and rate constants to some extent. Based on the G4-calibrated thermodynamic parameters, the temperature and pressure dependence of the rate constants and the two- and three-parameter Arrhenius coefficients for all reactions considered here have been determined by using the transition state theory (TST) and Rice-Ramsperger-Kassel-Marcus (RRKM) methods. The present results provide a comprehensive understanding of the mechanisms and kinetics of the LTO process of the 2-MF biofuel.

Extensive Theoretical Study of the Thermochemical Properties of Unsaturated Hydrocarbons and Allylic and Super-Allylic Radicals: The Development and Optimization of Group Additivity Values

In this study, the thermochemistry of C₂-C₇ unsaturated hydrocarbons (22 alkene and 6 diene molecules) and 16 allylic and 5 super-allylic radicals is determined using high-accuracy quantum chemistry calculations. In addition, the group additivity values (GAVs) of a total of 19 relevant groups are systematically optimized on the basis of the calculated thermochemistry of species clusters. The M06-2X method using the 6-311++G(d,p) basis set is used for the geometry optimizations, vibrational frequency calculations, and internal rotation scans for lower-frequency modes. The composite compound methods, CBS-APNO, G3, and G4, are utilized to derive the average atomization formation enthalpies. The entropy and temperature-dependent heat capacity values of all species are calculated using statistical thermodynamics in MultiWell. These results are in good agreement with literature data. A GAVs optimization is performed on the basis of a statistical analysis: a Bland-Altman plot, which is employed to visualize the agreement between the results from the quantum chemical calculations and the GA method. It is found that the 298 K entropies of the CD/C2, C/CD2/H2, C/C/CD2/H, and C/CD3/H groups disagree by more than 5 cal K^-1 mol^-1 compared to existing values, while the values for the ALLYLS and ALLYLT radical groups also differ by ∼2.4 and 4.1 cal K^-1 mol^-1, respectively. The 298 K formation enthalpies of the C/CD2/H2, C/C/CD2/H, C/CD3/H, and ALLYLT groups are modified by more than 1 kcal mol^-1, compared to existing values. The updated GAVs can be used with increased confidence to estimate the thermochemical properties of combustion-relevant unsaturated hydrocarbon molecules and their radicals which are critical for the development of accurate chemical kinetic models describing the pyrolysis and oxidation of hydrocarbon and oxygenated hydrocarbon fuels.