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,infin; = 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_rH°₁ (0 K)= -(146±10)kJ/mol and for C₂H₄OH of Δ_rH°₂ (0 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±0.2exp ((17560±1200) K/T) bar^-1 and K_2,eq = 2.1×10^-2(T/K)^-0.95±0.1× exp((14780±720) K/r) bar^-1, respectively.