Unimolecular Decomposition of 1,3-Dioxetane and 1,3,5-Trioxane Derivatives: Fast and Accurate Estimation of Kinetic Parameters

Abstract

Oxymethylene ethers (OMEs) are a high-potential novel type of e-fuel, which could play an important role in the transition toward a sustainable transportation sector. During the low-temperature oxidation of these OMEs, various cyclic ethers, such as 1,3-dioxetane and 1,3,5-trioxane derivatives, can be formed. However, the current lack of accurate reaction rate coefficients for the unimolecular decomposition of these cyclic ethers has been highlighted as an important obstacle in the development of detailed kinetic models for the low-temperature oxidation of larger OME molecules. In this light, a database with quantum chemical results has been constructed consisting of 34 4-membered ring and 34 6-membered ring cyclic ethers representing important functional groups present during the low-temperature oxidation of OMEs. This database has been used for the development of three estimation methods for the reaction rate coefficients, i.e., rate rules, the Evans−Polanyi relationship, and kinetic group additivity. Balancing performance and complexity, these three methods are currently still the most commonly used estimation methods by automatic kinetic model generation tools. Based upon an assessment of the results, kinetic group additivity was found to be the most accurate method, reaching kinetic accuracy, i.e., a maximum deviation of 1 order of magnitude, for all but one of the 4-membered rings and all but four of the 6-membered ring cyclic ethers. Interactions between the terminal end of a substituent and the ring in combination with the relative stability of different isomers have been proposed to be the main reason for the significant variations between the reaction rate coefficients of the various species. The kinetic parameters can be easily used for the development of detailed kinetic models for larger OMEs based on first principles.

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

“…A larger ring structure introduces more organization into the molecule than a smaller ring structure, and this leads to different impacts on the entropy and heat capacities. The disability of the group additivity theory to describe intramolecular interactions was similarly found to be the main source of errors during the development of a kinetic group additivity scheme for the reaction rate coefficients of the unimolecular decomposition of cyclic ethers . From the distribution of the differences, no systematic deviations can be observed for any of the thermodynamic properties.…”