Reactions of H₃O⁺ ions with free atoms of copper and with molecules of CuOH in gaseous flames

Abstract

Charge-exchange and proton-transfer reactions of the H₃O⁺ ion with atoms and compounds of copper have been investigated in the burnt gases of premixed flames of H₂+O₂+N₂ at atmospheric pressure. The concentrations of ions in these flames were determined using a quadrupole mass spectrometer. The reactions responsible for the production of copper-containing ions were found to be: with the reaction probably involving a third body, being in equilibrium. Measurements of the concentrations of all ions of copper allowed the rate constants for reactions (13) and (18) to be measured between 1820 and 2400 K. The rate constants derived for the exothermic proton-transfer reaction (13), between an ion and a molecule with a dipole moment, are large (ca. 3.5×10^-9 ions^-1 ml s^-1 at 2000 K) and show a negative temperature coefficient with k₁₃ varying as T^-2.0±1.0. The rate constant for the endothermic, ion–atom reaction (18) is somewhat smaller (ca. 4×10^-11 ions^-1 ml s^-1 at 2000 K) and exhibits a positive temperature coefficient; the associated activation energy is the endothermicity. It is noteworthy that the products of reaction (18) are those that minimise ΔH for the process. In addition, the equilibrium constant for reaction (11) and the enthalpy of hydration of Cu⁺ were derived by observing the ratio of the two copper-containing ions under different sampling conditions in two flames. The first hydration energy of Cu⁺ in the gas phase is determined to be -80±30 kJ mol^-1. The fact that CuOH⁺ was not detected in any of these flames shows that the ionisation potential of CuOH is larger than 745 kJ mol^-1. The proton affinity of CuOH is measured to be 891±35 kJ mol^-1.