Shock tube measurements of high temperature rate constants for OH with cycloalkanes and methylcycloalkanes

Abstract

High temperature experiments were performed with the reflected shock tube technique using multi-pass absorption spectrometric detection of OH radicals at 308 nm. The present experiments span a wide T-range, 801–1347 K, and represent the first direct measurements of the title rate constants at T > 500 K for cyclopentane and cyclohexane and the only high temperature measurements for the corresponding methyl derivatives. The present work utilized 48 optical passes corresponding to a total path length ∼4.2 m. As a result of this increased path length, the high [OH] detection sensitivity permitted unambiguous analyses for measuring the title rate constants. The experimental rate constants in units, cm 3 molecule −1 s −1, can be expressed in Arrhenius form as k OH + Cyclopentane = ( 1.90 ± 0.30 ) × 10 − 10 exp ( − 1705 ± 156 K / T ) ( 813–1341 K ) , k OH + Cyclohexane = ( 1.86 ± 0.24 ) × 10 − 10 exp ( − 1513 ± 123 K / T ) ( 801–1347 K ) , k OH + Methylcyclopentane = ( 2.02 ± 0.19 ) × 10 − 10 exp ( − 1799 ± 96 K / T ) ( 859–1344 K ) , k OH + Methylcyclohexane = ( 2.55 ± 0.30 ) × 10 − 10 exp ( − 1824 ± 114 K / T ) ( 836–1273 K ) . These results and lower- T experimental data were used to obtain three parameter evaluations of the experimental rate constants for the title reactions over an even wider T-range. These experimental three parameter fits to the rate constants in units, cm 3 molecule −1 s −1, are k OH + Cyclopentane = 1.390 × 10 − 16 T 1.779 exp ( 97 K / T ) cm 3 molecule − 1 s − 1 ( 209–1341 K ) , k OH + Cyclohexane = 3.169 × 10 − 16 T 1.679 exp ( 119 K / T ) cm 3 molecule − 1 s − 1 ( 225–1347 K ) , k OH + Methylcyclopentane = 6.903 × 10 − 18 T 2.148 exp ( 536 K / T ) cm 3 molecule − 1 s − 1 ( 296–1344 K ) , k OH + Methylcyclohexane = 2.341 × 10 − 18 T 2.325 exp ( 602 K / T ) cm 3 molecule − 1 s − 1 ( 296–1273 K ) . High level electronic structure methods were used to characterize the first three reactions in order to provide reliable extrapolations of the rate constants from 250–2000 K. The results of the theoretical predictions for OH + cyclohexane and OH + methylcyclopentane were sufficient to make a theoretical prediction for OH + methylcyclohexane. The present recommended rate expressions for OH with cyclohexane, and methylcyclohexane, give rate constants that are 15–25% higher (over the T-range 800–1300 K) than the rate constants utilized in recent modeling efforts aimed at addressing the oxidation of cyclohexane and methylcyclohexane. The current measurements reduce the uncertainties in rate constants for the primary cycloalkane consumption channel in a high temperature oxidation environment.