High temperature rate constants for OH+ alkanes

Abstract

Rate constants for H-atom abstractions by OH radicals from a series of alkanes (propane, n-butane, i-butane and neo-pentane) have been measured at high temperatures with the reflected shock tube technique using multi-pass absorption spectrometric detection of OH radicals at 308nm. The experiments represent the first direct measurements of these rate constants at T>1000K and span a wide T-range, 797–1259K.The present work utilized 80 optical passes corresponding to a total path length of ∼7m. As a result of this increased path length, the high [OH] detection sensitivity permitted pseudo-first-order analysis for unambiguously measuring the total rate constants. The experimental rate constants can be represented in Arrhenius form as,kC3H8+OH=6.671×10-11exp(-1543K/T)cm3molecule-1s-1(797–1248K)kn-C4H10+OH=9.674×10-11exp(-1569K/T)cm3molecule-1s-1(800–1236K)ki-C4H10+OH=9.114×10-11exp(-1654K/T)cm3molecule-1s-1(846–1221K)kneo-C5H12+OH=1.060×10-10exp(-1947K/T)cm3molecule-1s-1(841–1259K)The present results have been combined with prior lower-T measurements to generate three-parameter rate expressions that adequately represent the available direct measurements (within 25%) over a wide temperature regime (250–1250K). High-level ab-initio electronic structure theory computations of the molecular properties of reactants, products and transition states have been used to estimate theoretical rate constants with conventional transition state theory (CTST). The theoretical rate constants are excellent representations of the available experimental data (deviations less than 25%) and thereby offer a reliable method for extrapolation to higher-T as well as for extracting branching ratios for primary, secondary and tertiary abstractions.