Geometry of the magic number H+(H2O)21 water cluster by proxy

Abstract

Abundance mass spectra, obtained upon carefully electrospraying solutions of tert-butanol (TB) in water into a mass spectrometer, display a systematic series of peaks due to mixed H⁺(TB)_m(H₂O)_n clusters. Clusters with m + n = 21 have higher abundance (magic number peaks) than their neighbours when m ≤ 9, while for m > 9 they have lower abundance. This indicates that the mixed TB–H₂O clusters retain a core hydrogen bonded network analogous to that in the famous all-water H⁺(H₂O)₂₁ cluster up to the limit m = 9. The limiting value corresponds to the number of dangling O–H groups pointing out from the surface of the degenerated pentagonal dodecahedral, considered to be the lowest energy form of H⁺(H₂O)₂₁; the experimental findings therefore support this geometry. The experimental findings are supported by ab initio quantum chemical calculations to provide a consistent framework for the interpretation of these kinds of experiments.