Exploring the Ab Initio Kinetics of Trimethyl Phosphite

Trimethyl phosphite (TMPI) is an organophosphorus compound of growing interest in the contexts of fire safety and energetic materials. Yet, its gas-phase combustion kinetics remain largely underexplored. We develop a TMPI kinetic mechanism from first-principles quantum chemistry and master-equation (RRKM/MESS) calculations, supported by reactive molecular dynamics (ReaxFF-MD) to map early time bond activation and product growth. The potential-energy surfaces include C-O and P-O homolysis, hydrogen-atom abstraction (HAA) by Ḣ, ȮH, HȮ₂, ĊH₃, and CH₃Ȯ, and O₂, intramolecular H-transfer, and key association or isomerization steps. Thermochemistry (ΔHf°, S, c_p) and NASA polynomials are provided for all P-bearing intermediates. The model reproduces the expected Arrhenius behavior of ignition delay times (IDTs) for TMPI/air across a temperature range of 900-1500 K and pressures of 1 and 10 bar, with φ values ranging from 0.5 to 2.0. Increasing temperature and pressure shorten the IDT, with richer mixtures igniting faster. Sensitivity and flux analyses identify high-temperature chain branching (H + O₂ ⇌ O + OH) and control of the HO₂/OH pools as primary rate-controlling features, while TMPI-radical reactions that convert radicals to stable products inhibit ignition. Flux maps show HAA-initiated TMPI_R as the universal entry to the radical pool and reveal PO₂ as a central hub that feeds PO, HOPO/HOPO₂, and ultimately PO₃. Hybrid NVT+NVE MD trajectories further indicate an earlier onset of decomposition under adiabatic conditions, consistent with the rapid amplification of radicals once local hot spots are not thermostat-damped. The resulting mechanism and thermochemical set provide a consistent foundation for modeling phosphite oxidation and for comparing phosphite, phosphate, and phosphonate chemistries in fire-inhibition strategies.