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

FNO Bruce and X Bai and SY Cheng and X Wang and F Qin and CW Zhou and H Curran and Y Li, JOURNAL OF PHYSICAL CHEMISTRY A, 129, 10673-10693 (2025).

DOI: 10.1021/acs.jpca.5c06066

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 O-2 and radicals (H, OH, HO2, CH3, and CH3O) were evaluated using the Master Equation System Solver (MESS). The trend in reactivity based on forward barrier heights follows the order OH < H < CH3O < CH3 < HO2 < O-2. The OH radicals showed the lowest activation barrier (<1 kcal/mol) and the highest branching ratio at low temperatures. In contrast, the abstraction with H 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.

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