Investigation of cyclopentene + OH and cyclopentene thermal decomposition reactions

This study presents the first high-temperature measurements of cyclopentene+OH→k₁Products reaction over 856 – 1258 K and 1.03 – 4.33 bar. The determined rate coefficients of k₁ exhibited Arrhenius temperature dependence with an activation energy ∼ 3.3 kcal mol⁻¹ and negligible pressure dependence. Rate coefficients of OH + cyclopentene are faster than OH + 1-pentene but comparable to OH + 2-pentene, highlighting the significance of allylic C[sbnd]H bonds in these reactions. At 1258 K, our rate coefficient is ∼ 30 % slower than the average room-temperature measurement, indicating the role of OH-addition pathways at low temperatures. Site-specific rate coefficients and branching ratios are determined, with H-abstraction at the allylic C[sbnd]H site accounting for ∼68 % of the reaction and alkylic C[sbnd]H bond contributing ∼22 %. Our measured k₁ may be given as (unit: cm³molecule⁻¹s⁻¹): k₁=1.64×10⁻¹⁰e^{([Formula presented])} Cyclopentene decomposition reaction (cyclopentene→k₂cyclopentadiene+H₂) is studied over 1186–1472 K and 1.17 – 1.38 bar. Time-resolved yields of the product, cyclopentadiene, are monitored using a sensitive UV laser diagnostic near 220 nm. Absorption cross-sections of cyclopentadiene, measured in two separate mixtures, showed weak positive temperature dependence (8.7 – 9.8 × 10⁻¹⁸ cm²molecule⁻¹) over 961 – 1326 K. Our rate coefficients of k₂ exhibited a positive Arrhenius temperature dependence with an activation energy of ∼ 54.7 kcal mol⁻¹. At 1300 K, the decomposition of cyclopentene proceeds 5.5 and 28 times faster than that of cyclopentadiene and cyclopentane, respectively. Our measured k₂ may be expressed as (unit: s⁻¹): k₂=2.55×10¹²e^{(([Formula presented])} The pentagonal structure of cyclopentene influences its H-abstraction and decomposition reactions distinctively. While H-abstraction by OH follows a mechanism akin to cyclohexene and linear alkenes, with comparable rate coefficients at allylic C[sbnd]H bonds, the penta-ring structure significantly impacts the decomposition mechanism. Cyclopentene decomposition predominantly produces cyclopentadiene and H₂ in a ring-conserving manner, differing from the cleavage of allylic-alkylic C[sbnd]C bonds in the decompositions of 1-alkenes and cyclohexene.