Protonated thiohypofluorous acid, FSH2+. Theoretically predicted to be stable and experimentally observed in the gas phase

Protonated FSH has been theoretically investigated at the Gaussian-2 level of theory. The S-protonated isomer 1 was found to be more stable than the fluorine-protonated isomer 3 by 28.6 kcal mol−1, and the proton affinity of FSH at the sulphur atom is computed as 162 kcal mol−1 at 298 K. The interconversion of the two ions occurs through the three-centre transition structure 6, less stable than 1 by 53 kcal mol−1. The experimental observation of gaseous FSH2+ ions, obtained from the ionization of a mixture of H2S and NF3, is fully consistent with the theoretical prediction of at least one deep energy minimum on the potential energy surface and corroborates and extends previous experimental evidence for stable H2SF+SbF6− salt in anhydrous HF at 195 K.