Unraveling the thermal decomposition kinetics of triethyl phosphite

Triethyl phosphite (TEPI) is an organophosphorus compound of interest as a flame inhibitor and additive. Yet, its high-temperature decomposition pathways remain poorly characterized. This study integrates ab initio quantum chemical calculations, ReaxFF molecular dynamics (MD) simulations, and numerical kinetic modeling to investigate TEPI's thermal degradation. Molecular geometries and thermochemical parameters were obtained using M06-2X/6-311++G(d,p) and composite methods (G3B3, CBS-QB3, G3), supported by MP2/CCSD-based complete basis set (CBS) extrapolations. The C–O bond was identified as the weakest link (∼60 kcal/mol), consistent across electronic structure methods, and machine learning-based bond dissociation energy predictions (ALFABET). Despite variations in absolute bond dissociation energies among ReaxFF parameterizations, all methods confirmed C–O cleavage as the dominant initiation step. Reactive MD showed that TEPI is stable below 1500 K, with decomposition initiated by homolytic C–O scission yielding ethyl and an oxygen-centered radical (adjacent to the phosphorus atom). At 5000 K, over 240 intermediates were identified, including C₂H₅, CH₃, C₂H₄, CH₂O, PO, PO₂, and HOPO. A kinetic mechanism constructed from quantum chemistry and ReaxFF-MD-derived pathways was implemented in Chemkin to simulate ignition delay times (IDTs) for TEPI/oxidizer mixtures at equivalence ratios of 0.5, 1.0, and 2.0, pressures of 1 and 10 bar, and temperatures between 800 and 2000 K. The results show that IDTs shorten with increasing pressure and equivalence ratio, while decomposition is slowed under fuel-lean conditions due to radical scavenging by phosphorus-containing fragments. Flux and sensitivity analyses highlight initial C–O bond cleavage and subsequent PO/HOPO radical recombination as the dominant kinetic pathways. This work provides new mechanistic insights into TEPI decomposition as a transferable modeling framework for trivalent phosphorus-based compounds.