The proton affinities of CO and CO2 and the first hydration energy of gaseous H3O+ from mass spectrometric investigations of ions in rich flames of C2H2

Abstract

Fuel-rich oxy-acetylene flames, which are non-sooting and at 1 bar, have been sampled into a mass spectrometer. The flames used CO₂ as the diluent. The major positive ions in these flames were found to be CHO⁺, CHO₂⁺, C₃H₃⁺, H₃O⁺ and H₃O⁺·H₂O. The ions CHO⁺ and CHO₂⁺ participate in the equilibria: involving proton transfer between H₂O, CO and CO₂. Using CO₂ as diluent enabled CHO⁺ and CHO₂⁺ to be detected far downstream in the burnt gas. The use of triatomic CO₂ with its relatively low ratio of principal specific heats interestingly reduced the cooling experienced by a gas sample during its supersonic expansion into the mass spectrometer; also a sample from a flame with CO₂ as diluent spent less time in the supersonic expansion than a sample from a flame with Ar as diluent. In fact, flames with CO₂ as diluent were found to be unaffected by this usual sampling perturbation, whereby rapid equilibria adjust to the rapidly falling temperatures when a sample is expanded supersonically into a vacuum chamber. However, a sample from a flame is perturbed by being cooled somewhat by the cooler metal around the inlet orifice, through which the flame gas enters the mass spectrometer. After allowing for the effects of such cooling, the enthalpy change of reaction (8) was determined from measurements of [CHO⁺]/[H₃O⁺] to be − 120 ± 18 kJ mol⁻¹ at 0 K, corresponding to a proton affinity for CO of 576 ± 18 kJ mol⁻¹ at 298 K. Similarly the proton affinity of CO₂ was found to be 551 ± 20 kJ mol⁻¹ at 298 K. A study of [H₃O⁺ ·H₂O]/[H₃O⁺] along different flames enabled the enthalpy of mono-hydration of the ion H₃O⁺ to be measured as − 133 ± 30 kJ mol⁻¹ at 0 K; the rate constant for H₃O⁺ + H₂O + M → H₃O⁺ ·H₂O + M is ∽1 × 10⁻³⁰ molecule⁻² cm⁶ s⁻¹ at 2000 K.