Gaseous trihalogen cations. Formation, structure and reactivity of Cl3+ and Cl2F+ ions from a joint ab initio and FT-ICR study.

Gaseous Cl3+ ions were obtained by two convenient routes, namely Cl+ transfer to chlorine from Cl2H+ or Cl2.+ ions, whereas Cl2F+ was prepared upon fluorination of chlorine by XeF+. The structure and the stability of the trihalogen cations were investigated by reactive probing, utilizing FTICR mass spectrometry to survey their reactivity, in particular Cl+ transfer processes toward selected nucleophiles. The structure, relative stability, and dissociation enthalpies of Cl3+ and Cl2F+ were investigated by computational methods based on density functional theory up to the CCSD(T)/cc-pVQZ//B3LYP/6-311++G(3df, 3pd) level. The results show that an A1 singlet of C2v symmetry is the global minimum on the Cl3+ potential energy surface. Consistent with earlier results, the asymmetric bent [Cl−Cl−F]+, also an A1 singlet, is more stable by 44.3 kcal mol-1 at 298 K than the symmetric isomer of [Cl−F−Cl]+ connectivity. By combining theoretically computed dissociation enthalpies with available thermochemical data the heats of formation of Cl3+ and Cl2F+ cations, in their ground state, can be estimated to be 251.5 ± 5 and 245.0 ± 5 kcal mol-1 at 298 K. Comparison of the Cl+ and F+ binding energies to simple halogenated molecules shows an excellent linear correlation, which is not the case when the comparison is extended to the proton affinities. The different factors that influence the stability of protonated and halogenated adducts are briefly discussed.