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.