The High Pressure Range of the Addition of OH to C₂H₂ and C₂H₄

The kinetics of the bimolecular reactions OH+C 2 H 2 +M ⟺ C 2 H 2 OH+M (1) and OH+C 2 H 4 +M ⟺ C 2 H 4 OH+M (2) have been investigated over an extended pressure (1–130 bar) and temperature (300–800 K) range. The OH radicals have been generated by laser flash photolysis of suited precursors and their decays have been measured by saturated laser‐induced fluorescence (SLIF) under pseudo‐first‐order conditions. The pressure dependences have been analyzed by constructing falloff curves at fixed temperatures leading to reliable extrapolations towards the high pressure limiting rate constants k ∞ . In the given temperature range these rate constants are represented as k 1,∞ = 3.8×10 −11 exp (–910 K/ T ) cm 3 molecule −1 s −1 and as k 2,∞ = 1.0×10 −11 cm 3 molecule −1 s −1 . At temperatures above 700 K biexponential decay curves have been obtained. The chemical equilibria of reactions (1) and (2) could be determined. By a third law analysis the equilibrium constants have been evaluated with reaction enthalpies for the addition complex C 2 H 2 OH of δ I H o 1 (O K) = ‐(146 ± 10) kJ/mol and for C 2 H 4 OH of δ I H o 2 (O K) = ‐(123 ± 6) kJ/mol, respectively. The two equilibrium constants are given by K 1,eq = (5.4±2.2)×10 −2 ( T /K) −1.7±0.2 exp ((17560±1200) K/ T ) bar −1 and K 2,eq = 2.1 × 10 −2 (T/K) −95±0.1 × exp ((14780±720) K/ T ) bar −1 , respectively.