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, cp) 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.