Elemental chlorine and chlorine fluoride: theoretical and experimental proton affinity and the gas phase chemistry of Cl2H+ and FClH+ ions.

The chemistry of the title cations was investigated in the gas phase by Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry and by theoretical methods. Cl2H+ (1) was obtained from the protonation of Cl2 by gaseous Brønsted acids and the chlorination of HCl by Cl2F+ and XeCl+. The proton affinity (PA) of Cl2 measured by the FT-ICR equilibrium method is 131.4 ± 1 kcal mol-1 at 298 K, which compares with the 132.0 kcal mol-1 value computed at the CCSD(T) level of theory. 1 has the Cl−Cl−H connectivity, as indicated by its 2-fold reactivity as a Brønsted acid and a chlorinating agent, and confirmed by the theoretical results. The FClH+ cation was obtained from the fluorination of HCl by XeF+ and its most stable protomer (2) has the F−Cl−H connectivity, as shown by its behavior as a pure Brønsted acid and by the theoretical results that identify 2 as the global minimum, whereas protomer 3, having the H−F−Cl connectivity, is computed to be less stable by 5.0 kcal mol-1 at the CCSD(T) level of theory, respectively. For lack of sufficiently accurate experimental data, the best estimate of the PA of ClF is 121.0 kcal mol-1, computed at the CCSD(T) level of theory and referred to protonation of the Cl atom.