Kinetics of the Gas-Phase Reaction of Hydroxyl Radicals with Dimethyl Methylphosphonate (DMMP) over an Extended...

The kinetics of the reaction of hydroxyl radical (OH) with dimethyl methylphosphonate (DMMP, (CH₃O)₂CH₃PO) (reaction 1) OH + DMMP → products (1) was studied at the bath gas (He) pressure of 1 bar over the 295-837 K temperature range. Hydroxyl radicals were produced in the fast reaction of electronically excited oxygen atoms O(¹D) with H₂O. The time-resolved kinetic profiles of hydroxyl radicals were recorded via UV absorption at around 308 nm using a DC discharge H₂O/Ar lamp. The reaction rate constant exhibits a pronounced V-shaped temperature dependence, negative in the low temperature range, 295-530 K (the rate constant decreases with temperature), and positive in the elevated temperature range, 530-837 K (the rate constant increases with temperature), with a turning point at 530 ± 10 K. The rate constant could not be adequately fitted with a standard 3-parameter modified Arrhenius expression. The data were fitted with a 5-parameter expression as: k₁ = 2.19 × 10^-14(T/298)^2.43exp(15.02 kJ mol^-1/RT) + 1.71 × 10^-10exp(-26.51 kJ mol^-1/RT) cm³molecule^-1s^-1 (295-837 K). In addition, a theoretically predicted pressure dependence for such reactions was experimentally observed for the first time.