Can H₂ be Superacidic? A Computational Study of Triel-Bonded Brønsted Acids

The abundance of XIII group element compounds in science and industry together with their electron-deficient character gives rise to their influence on properties of the systems they interact with. This paper is an attempt to assess the strength, nature, and effect of formation of a triel bond on acidity. A wide set of Brønsted acids among others comprising hydrocarbons, halogen hydrides, and amines bonded with B, Al, and Ga trifluorides forming HX/TF₃ was selected for the research. Various computational approaches (e.g., MP2, GFN2-xTB, SAPT2 + 3(CCD)δ_MP2, quantum theory of atoms in molecules analysis, and density overlap regions indicator) are used to describe the triel-bonded systems. Among other things, it was found that the electrostatics may not be the dominant contribution to the triel binding in some cases. Additionally, it was established that even weak Brønsted acids such as C₂H₂ or H₂ may be superacidic if bonded to a Lewis acid (TF₃) that is strong enough. The calculations indicate a significant covalent character of some of the studied HX/TF₃ triel-bonded systems. Moreover, the effect of solvation of HX with TF₃ as well as that of the reverse process on the acidity of the resulting system is thoroughly described.