The Gas Phase Thermochemistry and the Hydrogen Atom Abstraction Reactions of Trimethyl Phosphite

Phosphorus-based compounds are increasingly studied for energy applications due to their unique functional properties and practical uses. However, phosphites, particularly in the gas phase, remain relatively underexplored. As a result, significant gaps persist in our understanding of their reactivity and stability. This study investigates trimethyl phosphite (TMPI) by combining quantum-chemical calculations to determine its thermochemical properties, reactive molecular dynamics (ReaxFF-MD) simulations to identify decomposition pathways, and kinetic calculations of hydrogen-atom abstraction (HAA) reactions. Molecular geometries were optimized at the M06-2X/6-311++G(d,p) level. Single-point energies were obtained using composite methods (G3, G3B3, CBS-QB3) and wavefunction-based calculations (MP2 and CCSD(T)). Composite results were averaged, while MP2 and CCSD(T) energies were extrapolated from cc-pVDZ to cc-pVQZ to approach the complete basis set (CBS) limit. The resulting averaged and CBS-extrapolated values were used to derive consistent bond dissociation energies (BDEs) and reaction energetics across various pathways. BDEs computed using the CBS-extrapolated method for C–O, C–H, and O–P bonds were 98.3, 56.4, and 93.9 kcal/mol, respectively. ReaxFF-MD-postulated decomposition pathways and product evolution trends corroborated key HAA and initiation pathways identified by quantum calculations. Six HAA reactions with O2 and radicals (Ḣ, ȮH, HȮ2, ĊH3, and CH3Ȯ) were evaluated using the Master Equation System Solver (MESS). The trend in reactivity based on forward barrier heights follows the order ȮH 3Ȯ 3 2 2. The ȮH radicals showed the lowest activation barrier (<1 kcal/mol) and the highest branching ratio at low temperatures. In contrast, the abstraction with Ḣ dominated the branching ratios at high temperatures. As experimental data on TMPI remain limited, these results provide insight into its gas-phase reactivity, which is relevant to combustion chemistry, flame inhibition, and the environmental degradation of organophosphorus compounds.