Superhalogens as building blocks of a new series of superacids

Abstract

Strong superacids, such as HSbF₆, typically consist of Brønsted/Lewis acid components (e.g., HF/SbF₅). Using MP2/6-311++G(d,p) calculations, we propose a new series of superacids by the protonation of B_nH_3n+1⁻ superhalogen anions. The resulting B_nH_3n+2 species behave as superacids for n ≥ 2 due to their smaller free energy of deprotonation (ΔG_dep = 292.7–258.8 kcal mol⁻¹) than that of H₂SO₄ (ΔG_dep = 302.2 kcal mol⁻¹). These B_nH_3n+2 superacids do not require Brønsted/Lewis acid components, whose gas phase acidity is governed by the vertical detachment energy (VDE) of the B_nH_3n+1⁻ superhalogens. The larger VDEs of B_nH_3n+1⁻ allow the B_nH_3n+2 species to easily release H⁺, increasing their acidic strength. We have also investigated other superhalogens, such as HSO₄⁻, BO₂⁻ and ClO₄⁻, and noticed that their VDEs follow the same order as that of the gas phase acidities of their protonated species, namely HClO₄ > H₂SO₄ > HBO₂.