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.

The reaction of the OH radical with isoprene, C ₅ H ₈ (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 ( k _1,298K = (9.90 ± 0.09) × 10 ^-11 cm ³ 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 + C ₅ H ₈ ⇌ HOC ₅ H ₈ 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 C ₁ and C ₄ 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 C ₁ adduct). Addition to the less stable C ₂ and C ₃ adducts is not important. The data above 700 K also indicated that a minor but significant direct abstraction channel, R1b, was also operating with k _1b = (1.3 ± 0.3) × 10 ^-11 exp(-3.61 kJ mol ^-1 / RT) cm ³ 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 k _1a ( T, [ M]). An analytical representation of k _1a ( T, [ M]) and k _-1a ( T, [ M]) is provided via a Troe expression. The reported experimental data (the sum of addition and abstraction), k ₁ ^∞ = (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) cm ³ molecule ^-1 s ^-1 , significantly extend the measured temperature range of R1.