Theoretical Unimolecular Decomposition Kinetics of Dimethyl, Diethyl, and Ethyl Methyl Carbonates and Their Fluorinated Counterparts

Dialkyl carbonates are relevant as sustainable fuel components and as lithium-ion battery electrolyte solvents, where their decomposition chemistry is important for combustion behavior, gas generation, and fire safety. In this work, the unimolecular decomposition kinetics of dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, and their fluorinated counterparts were investigated using the DLPNO–CCSD(T)/CBS // B97M-V/cc-pVTZ level of theory in combination with RRKM/Master Equation simulations. The resulting potential energy surfaces, bond dissociation energies, thermodynamic properties, and temperature- and pressure-dependent rate coefficients provide a consistent database for kinetic modeling of regular and fluorinated dialkyl carbonates. For the nonfluorinated compounds, the decomposition chemistry is governed by low-barrier molecular pathways, with diethyl carbonate (DEC) and ethyl methyl carbonate (EMC) predominantly forming alkoxyformic acids and ethylene, followed by rapid secondary decomposition to alcohols and carbon dioxide. Fluorination was found to generally strengthen C–H bonds and to suppress the dominant alkoxyformic acid-yielding pathways in fluorinated dialkyl carbonates with ethyl moieties. In contrast, fluorination of dimethyl carbonate decreases its stability because an energetically accessible F atom migration pathway accelerates fDMC decomposition by several orders of magnitude. Comparison with available experimental speciation data shows that the present rate coefficients perform comparably to existing models for DMC and DEC, have limited observable impact for EMC under the investigated high-temperature conditions, and improve CO predictions for fDEC pyrolysis. Among the investigated fluorinated carbonates, methyl 2,2,2-trifluoroethyl carbonate (fEMC) emerges as the most promising candidate in terms of combined unimolecular chemical stability and thermal buffering capacity, motivating further experimental and modeling studies of its potential as a safer lithium-ion battery solvent.