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

The overall rate constant for the reaction tert-butanol + OH -> products was determined experimentally behind reflected shock waves by using O-18-substituted tert-butanol (tert-butan(18)ol) and tert-butyl hydroperoxide (TBHP) as a fast source of (OH)-O-16. The data were acquired from 900 to 1200 K near 1.1 atm and are best fit by the Arrhenius expression 1.24 x 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 beta-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 beta-scission pathways of the tert-C4H8OH radical by comparing the measured pseudo-first-order decay rate of (OH)-O-16 in the presence of excess tert-butan(16)ol with the respective decay rate of (OH)-O-16 in the presence of excess tert-butan(18)ol. The two decay rates of (OH)-O-16 as a result of reactions with the two forms of tert-butanol differ by approximately a factor of 5 due to the absence of (OH)-O-16-producing pathways in experiments with tert-butan(18)ol. This indicates that 80% of the (OH)-O-16 molecules that react with tert-butan(16)ol will reproduce another (OH)-O-16 molecule through beta-scission of the resulting tert-(C4H8OH)-O-16 radical. (OH)-O-16 mole fraction time histories were measured using narrow-line-width laser absorption near 307 nm. Measurements were performed at the line center of the R-22(5.5) transition in the A-X(0,0) band of (OH)-O-16, a transition that does not overlap with any absorption features of (OH)-O-18, hence yielding a measurement of (OH)-O-16 mole fraction that is insensitive to any production of (OH)-O-18.