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

The kinetics of the bimolecular reactions OH+C₂H₂+M ⟺ C₂H₂OH+M (1) and OH+C₂H₄+M ⟺ C₂H₄OH+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) cm³ molecule⁻¹ s⁻¹ and as k_2,∞ = 1.0×10⁻¹¹ cm³ 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 C₂H₂OH of δ_IH^o₁ (O K) = -(146 ± 10) kJ/mol and for C₂H₄OH of δ_IH^o₂ (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.