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

Upon geometry optimization with no constraints (MP2/6-31G*), hydronium fluo roborate 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 /6-31G**) gave so mewhat different arrangements of clusters and ion geometries. The MP2/6-31G * geometry was symmetrical, with a boron to boron distance of 4.650 Angstro m and the closest interionic fluorine to fluorine distance of 2.783 Angstro m. Hydrogen bonds connect ions of opposite sign. Two groups of equivalent h ydrogens generate an AX(2)-type H-1 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 O-17 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 Angstrom (F ... F distances of 3.10, 3. 91 and 4.55 Angstrom), but decomposes to a solvated ion pair at B ... B of 6.50 Angstrom. Cooperation of acid sites inside porous solids should also f acilitate water protonation, to form double anion-hydronium ion clusters. W hether sites of equal strength protonate water at stoichiometric ratio depe nds upon the channel size and disposition of sites in it. The expectation o f narrowly defined spectral properties for protonated water in solid acids is not warranted. The protonating ability is determined by intrinsic streng th, distance between, and relative orientation of sites.