Kinetics of the Reaction of OH with Isoprene over a Wide Range of Temperature and Pressure Including Direct Observation of Equilibrium with the OH Adducts.

Abstract

The reaction of the OH radical with isoprene, C5H8 (R1), has been studied over the temperature range 298-794 K and bath gas pressures of nitrogen from 50 to 1670 Torr using laser flash photolysis (LFP) to generate OH and laser-induced fluorescence (LIF) to observe OH removal. Measurements have been made using both a conventional LFP/LIF apparatus and a new high pressure system. The measured rate coefficient at 298 K ( k1,298K = (9.90 ± 0.09) × 10-11 cm3 molecule-1 s-1) in the high pressure apparatus is in excellent agreement with the literature, confirming the accuracy of measurements made with this instrument. Above 700 K, the OH decays were no longer single exponentials due to regeneration of OH from adduct decomposition and the establishment of the OH + C5H8 ⇌ HOC5H8 equilibrium (R1a, R-1a). This equilibrium was analyzed by comparison to a master equation model of reaction R1 and determined the well depth for OH addition to carbon C1 and C4 to be equal to 153.5 ± 6.2 and 143.4 ± 6.2 kJ mol-1, respectively. These well depths are in excellent agreement with the present ab initio-CCSD(T)/CBS//M062X/6-311++G(3df,2p)-calculations (154.1 kJ mol-1 for the C1 adduct). Addition to the less stable C2 and C3 adducts is not important. The data above 700 K also indicated that a minor but significant direct abstraction channel, R1b, was also operating with k1b = (1.3 ± 0.3) × 10-11 exp(-3.61 kJ mol-1/ RT) cm3 molecule-1 s-1. Additional support for the presence of this abstraction channel comes from our ab initio calculations and from room-temperature proton transfer mass spectrometry product analysis. The literature data on reaction R1 together with the present data were assessed using master equation analysis, using the MESMER package. This analysis produced a refined data set that generates our recommended k1a( T, [ M]). An analytical representation of k1a( T, [ M]) and k-1a( T, [ M]) is provided via a Troe expression. The reported experimental data (the sum of addition and abstraction), k1∞ = (9.5 ± 0.2) × 10-11( T/298 K)-1.33±0.07 + (1.3 ± 0.3) × 10-11 exp(-3.61 kJ mol-1/ RT) cm3 molecule-1 s-1, significantly extend the measured temperature range of R1.