Equilibrium OH− + C2H2 ⇄ C2H− + H2O and the determination of Δ H°f,298 (C2H−)

The kinetics of the proton transfer reaction OH⁻ + C₂H₂ ⇄ C₂H⁻ + H₂O have been investigated at 296°K with the flowing afterglow technique. The rate constants were determined to be k_forward = (2.2±0.4) × 10⁻⁹ cm³ molecule⁻¹ sec⁻¹ and k_reverse = (9.3±3.7) × 10⁻¹⁵ cm³ molecule⁻¹ sec⁻¹. The ratio of rate constants, k_f/k_r = (2.4±1.0) × 10⁵, was found to be in agreement with the equilibrium constant, K = (2.5±1.0) × 10⁵, determined from equilibrium concentrations. The best estimate of the true value of K corresponds to a standard free energy change, ΔG°₂₉₆, of −7.3±0.2 kcal mole⁻¹. An estimate of the standard entropy change, ΔS°₂₉₈, of +3.6 cal deg⁻¹ mole⁻¹ leads to a standard enthalpy change, ΔH°₂₉₈, of −6.2±0.5 kcal mole⁻¹ and a value for ΔH°_f,298 (C₂H⁻) = 71.5±1.1 kcal mole⁻¹. O⁻ was observed to react rapidly with C₂H₂, k=(2.0±0.4) × 10⁻⁹ cm³ molecule⁻¹ sec⁻¹, principally by associative detachment and proton transfer. The reaction of ${\mathrm{O}}_2^{-}$ with C₂H₂ was observed to be slow, k ≤ 1 × 10⁻¹² cm² molecule⁻¹ sec⁻¹.