C3H+3 in flames and the proton affinity of C3H2

C3H+3 is prominent in the reaction zone of a methane + argon diffusion flame, but it is displaced by NH+4 when ammonia is added to the flame. These results may be explained by considering the equilibria in the two proton-transfer reactions H3O++ C3H2= C3H+3+ H2O (A), NH+4+ C3H2= C3H+3+ NH3. (B). In reaction (A), the proton affinity of C3H2 is very much greater than that of H2O, so that H3O+ is only formed in substantial quantities when the concentration of H2O is several orders of magnitude greater than that of C3H2. In reaction (B), the higher proton affinity of NH3 ensures that NH+4 dominates the reaction zone. Ion cyclotron resonance (i.c.r.) experiments indicate that the proton affinity of C3H2 is more than 65 kJ mol–1 greater than that of ammonia. The concentration of C3H2 in the reaction zone was estimated to be 7 × 1012 cm–3, lower than the detection limit of mass spectrometry.