The stability of the hydronium tetrafluoroborate dimer determined by theoretical calculations. Implications for water protonation in nonpolar solvents and on solid acids

Abstract

Upon geometry optimization with no constraints (MP2/6-31G*), hydronium fluoroborate decomposed, but the dimer (H₃O⁺·BF₄⁻)₂ was stable. In the dimer, the anions face each other and create a cavity containing the two cations. Density functional theory calculations (B3LYP/6-31G* to /6-31++G**) gave somewhat different arrangements of clusters and ion geometries. The MP2/6-31G* geometry was symmetrical, with a boron to boron distance of 4.650 Å and the closest interionic fluorine to fluorine distance of 2.783 Å. Hydrogen bonds connect ions of opposite sign. Two groups of equivalent hydrogens generate an AX₂-type ¹H NMR spectrum, as found for a hydronium salt in Freon solution; dimeric clusters should also be present in nonpolar solvents. In concentrated aqueous solution, however, the ¹⁷O NMR spectrum indicates the presence of ion pairs. The double cluster is stable at B···B distances of 5.00, 5.50 and 6.00 Å (F···F distances of 3.10, 3.91 and 4.55 Å), but decomposes to a solvated ion pair at B···B of 6.50 Å. Cooperation of acid sites inside porous solids should also facilitate water protonation, to form double anion–hydronium ion clusters. Whether sites of equal strength protonate water at stoichiometric ratio depends upon the channel size and disposition of sites in it. The expectation of narrowly defined spectral properties for protonated water in solid acids is not warranted. The protonating ability is determined by intrinsic strength, distance between, and relative orientation of sites.