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 < Ḣ < CH₃Ȯ < ĊH₃ < HȮ₂ < O₂. 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.