Direct measurements of channel specific rate constants in OH + C3H8 illuminates prompt dissociations of propyl radicals

Abstract

OH + molecules are an important class of reactions in combustion and atmospheric chemistry. Consequently, numerous studies have measured rate constants for these processes over an extended temperature range. A large majority of these experimental studies have utilized the decay of [OH] profiles (monitored either by absorption or laser-induced fluorescence) to obtain total rate constants. However, there are limited direct measurements of channel specific rate constants in this important class of reactions, particularly at combustion relevant temperatures. In the present experiments, we have directly measured site-specific rate constants for abstraction of the secondary CH bond in OH + C3H8 at high temperatures. Atomic resonance absorption spectrometry (ARAS) was used to monitor the formation of H-atoms from shock-heated mixtures of tert-butylhydroperoxide and C3H8 at high temperatures. Simulations for the experimental H-atom profiles are sensitive only to abstraction of the secondary CH bond leading to unambiguous measurements of the rate constants for this reaction. Over the T-range, 921 K < T < 1146 K, rate constants from the present experiments for OH + C3H8 → H2O + i-C3H7 can be represented by the Arrhenius expression,k=(3.935±1.387)×10−11exp(−1681±362K/T)cm3molecule−1s−1Simulations of the lower temperature data (T < 1000 K) indicate that the H-atom profiles are also influenced to a minor extent by the thermal dissociation of iso-propyl, i-C3H7 → H + C3H6, at short time-scales. Direct dynamics calculations were performed to examine in greater detail the potential role of prompt dissociations of i-C3H7 and n-C3H7 (formed from the title reaction) in interpreting the lower temperature (< 1000 K) data from the present work. These simulations suggest that prompt dissociation of propyl radicals does not influence the present experimental observations but has a minor influence on higher temperature combustion simulations.