Shock Tube Measurements of the tert-Butanol + OH Reaction Rate and the tert-C4H8OH Radical β-Scission Branching Ratio Using Isotopic Labeling

Abstract

The overall rate constant for the reaction tert-butanol + OH → products was determined experimentally behind reflected shock waves by using (18)O-substituted tert-butanol (tert-butan(18)ol) and tert-butyl hydroperoxide (TBHP) as a fast source of (16)OH. The data were acquired from 900 to 1200 K near 1.1 atm and are best fit by the Arrhenius expression 1.24 × 10(-10) exp(-2501/T [K]) cm(3) molecule(-1) s(-1). The products of the title reaction include the tert-C4H8OH radical that is known to have two major β-scission decomposition channels, one of which produces OH radicals. Experiments with the isotopically labeled tert-butan(18)ol also lead to an experimental determination of the branching ratio for the β-scission pathways of the tert-C4H8OH radical by comparing the measured pseudo-first-order decay rate of (16)OH in the presence of excess tert-butan(16)ol with the respective decay rate of (16)OH in the presence of excess tert-butan(18)ol. The two decay rates of (16)OH as a result of reactions with the two forms of tert-butanol differ by approximately a factor of 5 due to the absence of (16)OH-producing pathways in experiments with tert-butan(18)ol. This indicates that 80% of the (16)OH molecules that react with tert-butan(16)ol will reproduce another (16)OH molecule through β-scission of the resulting tert-C4H8(16)OH radical. (16)OH mole fraction time histories were measured using narrow-line-width laser absorption near 307 nm. Measurements were performed at the line center of the R22(5.5) transition in the A-X(0,0) band of (16)OH, a transition that does not overlap with any absorption features of (18)OH, hence yielding a measurement of (16)OH mole fraction that is insensitive to any production of (18)OH.