Estimating Thermodynamic Properties of Oxymethylene-Ether-like Species Using Group Additivity

Developing detailed kinetic models for pyrolysis and combustion requires the availability of accurate thermodynamic properties. As it is time-consuming to perform quantum chemical calculations for all species in a kinetic model, one often relies on fast estimation methods, such as group additivity theory. Oxymethylene ethers are a high-potential class of oxygenated e-fuels for which detailed kinetic models for pyrolysis and combustion are still lacking. The available group additivity schemes in the literature do not provide accurate thermodynamic properties for oxymethylene-ether-like species. Therefore, a new group additivity scheme with 24 molecular groups is developed based on a quantum chemical data set with 121 species at the CBS-QB3 level of theory. These species contain a variety of oxygenated functionalities, such as ether, hydroxyl, carbonyl, and carboxyl groups. The group additivity scheme enables the estimation of the standard enthalpy of formation and standard entropy at 298 K, in addition to standard heat capacities over the temperature range of 300–3000 K. For a test set of 20 species, the estimated enthalpies of formation at 298 K have a mean absolute deviation (MAD) of 3.1 kJ mol–1 compared to the quantum chemically determined values. Similarly, the MAD values amount to 7.3 and 5.2 J mol–1 K–1 for the entropy at 298 K and heat capacity at 300 K, respectively. Due to the inability to accurately describe the impact of intramolecular interactions, chemical accuracy for the entropy and heat capacities, defined as an error of 4 J mol–1 K–1, cannot be reached. For another test set of 10 species, the discrepancy between experimentally determined and group additivity-estimated enthalpies of formation reaches chemical accuracy with an MAD of 2.5 kJ mol–1. The developed group additivity scheme outperforms the other ones available in the literature.