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

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.