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

Subgraph Isomorphic Decision Tree to Predict Radical Thermochemistry with Bounded Uncertainty Estimation

Detailed chemical kinetic models offer valuable mechanistic insights into industrial applications. Automatic generation of reliable kinetic models requires fast and accurate radical thermochemistry estimation. Kineticists often prefer hydrogen bond increment (HBI) corrections from a closed-shell molecule to the corresponding radical for their interpretability, physical meaning, and facilitation of error cancellation as a relative quantity. Tree estimators, used due to limited data, currently rely on expert knowledge and manual construction, posing challenges in maintenance and improvement. In this work, we extend the subgraph isomorphic decision tree (SIDT) algorithm originally developed for rate estimation to estimate HBI corrections. We introduce a physics-aware splitting criterion, explore a bounded weighted uncertainty estimation method, and evaluate aleatoric uncertainty-based and model variance reduction-based prepruning methods. Moreover, we compile a data set of thermochemical parameters for 2210 radicals involving C, O, N, and H based on quantum chemical calculations from recently published works. We leverage the collected data set to train the SIDT model. Compared to existing empirical tree estimators, the SIDT model (1) offers an automatic approach to generating and extending the tree estimator for thermochemistry, (2) has better accuracy and R2, (3) provides significantly more realistic uncertainty estimates, and (4) has a tree structure much more advantageous in descent speed. Overall, the SIDT estimator marks a great leap in kinetic modeling, offering more precise, reliable, and scalable predictions for radical thermochemistry.

Probing the Chemistry of Sulfurous Pollutants: Accurate Thermochemistry Determination of Extensive Sulfur-Containing Species

Sulfur-containing fuels, such as petroleum fuels, natural gas, and biofuels, produce SO2, SO3, and other highly toxic gases upon combustion, which are harmful to human health and the environment, making it essential to understand their thermochemical properties. This study used high-level quantum chemistry calculations to determine thermodynamic parameters, including entropy, enthalpy, and specific heat capacity for an extensive set of sulfur-containing species. The B3LYP/cc-pVTZ level of theory was used for geometry optimization, vibration frequency, and dihedral scan calculations. To determine an appropriate ab initio method for energy calculation, the Bland-Altman diagram, a statistical analysis method, was employed to visualize the 298 K enthalpy value between experimental data and three sets of ab initio methods: G3, CBS-QB3, and the average of G3 plus CBS-QB3. The CBS-QB3 method exhibited the highest accuracy and was eventually selected for the energy calculation in this study. Thermochemical property parameters were then calculated with the MultiWell program suite for all these sulfur-containing species, and the results were in good agreement with the thermochemical data of organic compounds and the National Institute of Standards and Technology Chemistry WebBook databases. The thermochemical property database established in this study is essential to studying sulfur-containing species in desulfurization.

An extensive theoretical study on the thermochemistry of aromatic compounds: from electronic structure to group additivity values

An extensive and reliable database of thermodynamic properties of C₆-C₁₂ aromatic molecules is constructed by using quantum chemistry calculations. There are 101 molecules in total, which cover a variety of structures including mono-substituted, di-substituted, and bi-cyclic aromatics which can be important intermediates in the combustion of alkylbenzenes. Based on the database, a consistent set of Benson group additive values (GAV) and non-nearest neighbor interactions (NNI) is developed to extend the applicability of Benson's group additivity method for aromatic molecules. Meanwhile, GAVs of existing groups are also updated to improve their accuracy. Geometry optimizations, and vibrational frequency calculations are conducted at the M06-2X/6-311++G(d,p) level of theory. Internal rotor potentials for lower-frequency modes are obtained at the M06-2X/6-31G level of theory. G3 and G4 compound methods are used to derive the 0 K enthalpies of formation via the atomization approach. The entropy and temperature-dependent heat capacity values of all species are calculated via the Master Equation System Solver (MESS) code. This work also provides an extensive literature comparison to validate the calculated results, and good agreement is observed with literature data. The correction terms beyond a group range are explored. The NNIs of di-substituted aromatics with substituents including OH, CHO, and CH₃ groups are reported. Entropy reduction is observed in the molecules with two substituents in the ortho position, which mainly derives from the hindered internal rotations. In addition, ring strain corrections (RSC) of dicyclic aromatics are evaluated. The strain energies of molecules with a four-membered side ring are prominently large, as the bond length and bond angle distortions are severely restricted. Ring strain also plays a key role in the C-H bond strength associated with the benzylic carbons in dicyclic aromatics. The loss of a hydrogen atom can destroy the high ring-strain geometry leading to a large C-H bond energy.

Thermodynamic Properties: Enthalpy, Entropy, Heat Capacity, and Bond Energies of Fluorinated Carboxylic Acids

Fluorinated carboxylic acids and their radicals are becoming more prevalent in environmental waters and soils as they have been produced and used for numerous commercial applications. Understanding the thermochemical properties of fluorinated carboxylic acids will provide insights into the stability and reaction paths of these molecules in the environment, in body fluids, and in biological and biochemical processes. Structures and thermodynamic properties for over 50 species related to fluorinated carboxylic acids with two and three carbons are determined with density functional computational calculations B3LYP, M06-2X, and MN15 and higher ab initio levels CBS-QB3, CBS-APNO, and G4 of theory. The lowest energy structures, moments of inertia, vibrational frequencies, and internal rotor potentials of each target species are determined. Standard enthalpies of formation, Δ_fH₂₉₈^°, from CBS-APNO calculations show the smallest standard deviation among methods used in this work. Δ_fH₂₉₈^° values are determined via several series of isodesmic and/or isogyric reactions. Enthalpies of formation are determined for fluorinated acetic and propionic acids and their respective radicals corresponding to the loss of hydrogen and fluorine atoms. Heat capacities as a function of temperature, C_p(T), and entropy at 298 K, S₂₉₈^°, are determined. Thermochemical properties for the fluorinated carbon groups used in group additivity are also developed. Bond dissociation energies (BDEs) for the carbon-hydrogen, carbon-fluorine, and oxygen-hydrogen (C-H, C-F, and O-H BDEs) in the acids are reported. The C-H, C-F, and O-H bond energies of the fluorinated carboxylic acids are in the range of 89-104, 101-125, and 109-113 kcal mol^-1, respectively. General trends show that the O-H bond energies on the acid group increase with the increase in the fluorine substitution. The strong carbon fluorine bonds in a fluorinated acid support the higher stability of the perfluorinated acids in the environment.

Gas-to-Liquid Phase Transition of PAH at Flame Temperatures

Significant evidence has shown that soot can be formed from polycyclic aromatic hydrocarbon (PAH) in combustion environments, but the transition of high molecular PAH from the gas phase to soot in a liquid or solid state remains unclear. In this study, the relationships between the boiling points of various planar PAHs and their thermodynamic properties are systematically investigated, to find a satisfactory marker for the phase transition event. Temperature-dependent thermodynamic properties, including entropy, specific heat capacity, enthalpy, and Gibbs free energy, are simultaneously calculated for PAHs, using density functional theory and three composite compound methods. Comparison of the results indicates that the individual G3 method, plus an atomization reaction approach, produces the most accurate thermochemistry parameters. Compared to entropy, enthalpy, and Gibbs free energy, the specific heat capacity at 298 K is found to be a better marker for the boiling point of PAHs due to the observed linear correlation, predictable characteristics, and fidelity of accuracy as a function of temperature. The correlation equation Y = 10.996X + 122.111 is proposed (where Y is the boiling temperature (K) and X is C_p at 298 K (cal/K/mol)). The standard deviation is as low as 16.7 K when comparing the calculated boiling points and experimentally determined values for 25 different aromatic species ranging from benzene to ovalene (C₃₂H₁₄). The effects of carbon number, structural arrangement, and partial pressure on the boiling point of large planar PAH are discussed. The results reveal that the carbon number in large planar PAH is the dominant factor determining its boiling points. It is shown that PAHs containing about 60-65 carbon atoms are likely to exist as liquids in flames, although the partial pressure of such species is very low.

Thermochemistry of Intermediates and Products in the Oxidation Reaction of 1,1,2-Trifluoroethene via OH Radical

Density functional theory (DFT) and composite ab initio based calculations are performed on trifluoroethane along with intermediate radicals, parent molecules of the radicals, and products related to the reaction of hydroxyl radical with 1,1,2-trifluoroethene, as a reference for hydrofluoroolefins (HFO). Potential energy barriers for internal rotations have been computed. Calculated torsional potentials are incorporated into the determination of entropy, S°₂₉₈, and heat capacities as a function of temperature, C_p(T), for each target molecule. Six isodesmic or isogyric reactions and five calculation methods are used to determine heats of formation at 298 K (Δ_fH₂₉₈) in kcal mol^-1 of each target species. The CBS-APNO method shows the best agreement with experimental data in comparisons from 16 reference reactions on Δ_rxnH of each method. The lowest configuration structures of each target species are reported. Intramolecular hydrogen bonds between the hydroxyl hydrogen atom and the fluorine atom on the adjacent carbon can stabilize molecules by up to 3 kcal mol^-1. R-OH bond dissociation energies are observed to increase with the number of fluorine atoms on the carbon connected to hydroxy group. Recommended Δ_fH₂₉₈ values in kcal mol^-1 derived from the most stable conformers are CF₂(OH)CH₂F (-213.0), CF₂(O^•)CH₂F (-148.6), CF₂(OH)C^•FH (-162.4), CHF₂CHFOH (-207.5), CHF₂C^•FOH (-158.3), C^•F₂CHFOH (-155.5), CHF₂CHFO^• (-150.4), CF₃CH₂OH (-212.5), and CF₃C^•HOH (-167.9).