Variable-temperature rate coefficients of proton-transfer equilibrium reaction C₂H₄ + H₃O⁺ ↔ C ₂H₅⁺ + H₂O measured with a coaxial molecular beam radio frequency ring electrode ion trap

The rate coefficients for the forward and reverse proton-transfer reactions C₂H₄ + H₃O⁺ ↔ C ₂H₅⁺ + H₂O are studied with respect to independent varied neutral molecule and ion temperatures. The measurements are performed using a coaxial molecular beam radio frequency ring electrode ion trap at trap temperatures down to 23 K and beam temperatures up to 450 K. The temperature-dependent rate coefficients suggest that in this temperature window, the reaction proceeds through a statistically equilibrated complex. In order to explain the observed rate coefficients, a new type of reaction temperature was defined in these studies that considered collisional and internal (rotational and vibrational) degrees of freedom of both H₃O⁺ and C₂H₄. The enthalpy and entropy of the equilibrium reaction deduced from a Van't Hoff plot are δH = (5.1 ± 0.5) kJ̇mol^-1 and δS = (-15.0 ± 0.9) J̇mol ^-1̇K^-1, respectively.