Kinetics of the Gas-Phase Reaction of Hydroxyl Radicals with Dimethyl Methylphosphonate (DMMP) over an Extended Temperature Range (273-837 K).

The kinetics of the reaction of hydroxyl radical (OH) with dimethyl methylphosphonate (DMMP, (CHO)CHPO) (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(D) with HO. The time-resolved kinetic profiles of hydroxyl radicals were recorded via UV absorption at around 308 nm using a DC discharge HO/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: k = 2.19 × 10(T/298)exp(15.02 kJ mol/RT) + 1.71 × 10exp(-26.51 kJ mol/RT) cmmolecules (295-837 K). In addition, a theoretically predicted pressure dependence for such reactions was experimentally observed for the first time.