Kinetics of the Gas-Phase Reaction of Hydroxyl Radicals with Trimethyl Phosphite (TMPi) over the 295-735 K Temperature Range.

The kinetics of the gas-phase reaction between hydroxyl radicals (OH) and trimethyl phosphite (TMPi, (CH3O)3P): (OH + TMPi → products (reaction 1) was investigated over the 295-735 K temperature range at a total pressure of 1 bar using helium as the bath gas. Hydroxyl radicals were generated in situ via the rapid reaction of electronically excited oxygen atoms, O(1D), with water vapor, providing a clean source of OH radicals. The time-resolved OH concentration profiles were monitored by ultraviolet absorption near 308 nm using a DC-discharge H2O/Ar low-pressure resonance lamp. The pseudo-first-order rate coefficients were obtained from exponential fits to the initial portions of the OH temporal profiles and converted to the bimolecular rate constants based on the TMPi concentrations. The reaction rate constant shows a pronounced negative temperature dependence across the studied temperature interval, with the reaction rate decreasing as the temperature increases. This behavior is consistent with that observed for related organophosphorus esters, including trimethyl phosphate and dimethyl methyl phosphonate, and supports a mechanistic interpretation involving a submerged transition state along the reaction coordinate. The measured rate constant is represented by the Arrhenius expression over the 295-735 K range: k1 = 1.27 × 10-11exp(619 K/T) cm3 molecule-1 s-1, corresponding to an apparent negative activation energy of -5.15 kJ mol-1 (-1.23 kcal mol-1). The magnitude and the negative temperature dependence of k1 suggest that attractive long-range interactions and barrierless or near-barrierless channels dominate the entrance dynamics. These results extend the kinetic database for OH reactions with reduced phosphorus compounds and provide constraints for detailed reaction mechanisms relevant to phosphorus-containing species in combustion and atmospheric oxidation environments.