Kinetics of the gas-phase reaction of hydroxyl radicals with trimethyl phosphate over the 273–837 K temperature range

Abstract

The kinetics of the reaction of hydroxyl radical (OH) with trimethyl phosphate (CH₃O)₃PO (TMP) (reaction (1)) OH + TMP → products (1) was studied at the bath gas (He) pressure of 1 bar over the 273–837 K temperature range. Hydroxyl radicals were produced in fast reactions of electronically excited oxygen atoms O(¹D) with either H₂O or H₂. Excited oxygen atoms O(¹D) were produced by photolysis of ozone, O₃, at 266 nm (4^th harmonic of Nd:YAG laser) over the 273–470 K temperature range and by photolysis of N₂O at 193 nm (ArF excimer laser) over the whole temperature range including the elevated temperature range 470–837 K. The reaction rate constant exhibits a V-shaped temperature dependence, negative in the low temperature range, 273–470 K (the rate constant decreases with temperature), and positive in the elevated temperature range, 470–837 K (the rate constant increases with temperature), with a turning point at 471 K. The rate constant could be fairly well fitted with the three parameter modified Arrhenius expression, k₁ = 7.52 × 10⁻¹⁸ (T/298)⁹ exp(34 367 J mol⁻¹/RT) cm³ per molecule per s (273–837 K). Previously, only one indirect experimental measurement at a single (ambient) temperature was available. The temperature dependence over an extended temperature range obtained in this study together with the peculiar V-shaped temperature dependence will have an impact on the modelling of the flame inhibition by phosphates as well on the further understanding of the mechanisms of elementary chemical reactions.