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 (4th 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.