The saturation of the gas phase acidity of nHF/AlF3 and nHF/GeF4 (n ¼ 1–6) superacids caused by increasing the number of surrounding HF molecules

Abstract

The acidic strength of selected Brønsted/Lewis superacids is evaluated on the basis of theoretical calculations carried out at the QCISD/6-311++G(d,p) level. The energies and Gibbs free energies of deprotonation processes for nHF/AlF3 and nHF/GeF4 (n ¼ 1–6) are found to depend on the number (n) of hydrogen fluoride molecules (playing a Brønsted acid role) surrounding the AlF3 and GeF4 Lewis acids. The successive attachment of HF molecules to either AlF3 or GeF4 gradually increases the acidity strength of the resulting superacid, which leads to the saturation achieved for 5–6 HF molecules interacting with either one of these Lewis acids. The importance of the microsolvation of the corresponding anionic species as well as the necessity of considering larger (more structurally complex) building blocks of superacids while predicting their acidity is indicated and discussed.