Ab Initio Calculations of the¹⁷O NMR Chemical Shift of Hydronium and Dihydroxonium Ions in Their Fluoroborates:  Comparison with Experiment

Abstract

The geometries of the hydronium and dihydroxonium cations in ion pairs with fluoroborate anions were examined by ab initio calculations at the MP2/6-31G* level. It was found that the representation of the hydronium ion in the field of an anion as an equilateral triangle, employed in the literature for the interpretation of low-temperature broad-band NMR spectra of water in solid acids, is an oversimplification, particularly for the composition H 5 O 2 + (dihydroxonium). Chemical shift calculations (DFT-GIAO-B3LYP at the dzvp, tzp, tz2p, and qz2p levels) were conducted for 17 O in H 3 O + ‚BF 4 -(1) and H 5 O 2 + ‚BF 4 -(2). The signal of 2 was predicted to appear at higher frequency (downfield) than the signal for 1. For experimental verification, the 17 O NMR spectra were recorded for various mixtures of hydronium fluoroborate and water. A nonmonotonic variation of the 17 O chemical shift with the increase in water content was observed; the signal moved first toward higher frequency and had the highest chemical shift for a water-to-hydronium ratio of 1:1 (H 5 O 2 + ), after which a monotonic variation toward lower frequency (upfield) was observed. Along both branches of the δ 17 O vs composition plot (H 3 O + ‚BF 4to H 5 O 2 + ‚BF 4and H 5 O 2 + ‚BF 4to H 2 O) the chemical shift variation was nonlinear. Thus, the experiments and the calculations were in qualitative agreement (the signal for 1 at lower frequency than the signal for 2), but the chemical shift difference predicted by the calculations was larger than the experimental result. Better agreement between the calculated and measured chemical shift differences is obtained for an orientation of ions in 2 with two fluorine atoms hydrogen bonded with the cation. Likewise, a better agreement is obtained for the pyramidal form than for the planar form of 1, in agreement with the geometry optimization results.

Tetrafluoroborate and Hexafluorophosphate Ions are not Interchangeable: A Density Functional Theory Comparison of Hydrogen Bonding

“…The rate of this hydrolysis reaction is strongly inhibited by the presence of tetrafluoroborate, but is not inhibited by hexafluorophosphate ,. A possible explanation for the inhibitory effect of BF 4 − is complexation of this ion with hydronium (H 3 O + ) ions, which has been suggested to occur in nonpolar, aprotic solvents, and supported by nuclear magnetic resonance (NMR) spectroscopy and density functional theory (DFT) calculations . If such clustering also occurs in polar, aprotic solvents like THF, it could inhibit the hydrolysis reaction by making both the co‐reactant (H 2 O) and catalyst (H 3 O + ) less available to the alkyl thiosulfate.…”

“…Geometry optimization was also performed for 1:1 complexes, BF 4 − •H 3 O + ( 10 ) and PF 6 − •H 3 O + ( 11 ), but unlike the stable dimers, these clusters decomposed into H 2 O, HF, BF 3 , and PF 5 (Figure ), consistent with the calculations of Farcasiu et. al ,. We note that such 1:1 complexes are likely intermediates en route to the dimers, and that the presence of solvent could serve to stabilize them relative to this type of decomposition.…”

“…The results of our geometry optimization for (BF 4 − •H 3 O + ) 2 agree with those reported by Farcasiu and coworkers . As shown in Figure , there are four hydrogen bonds formed between the H and F atoms in cluster 7 , each with a H•••F distance of 1.47 Å.…”

“…The previous computational work by Farcasiu and coworkers on complexation of tetrafluoroborate and hydronium ions in nonpolar solvents concluded that the most stable complex is dimeric and contains two of each ion arranged in a cyclic structure. We confirmed these results for tetrafluoroborate ion and extended the analysis to include potential H‐bonded complexes between hexafluorophosphate and hydronium ions.…”

The Combination of Superhalogens and Brønsted Acids HX (X = F, Cl, Br): An Effective Strategy for Designing Strong Superacids

“…Experimentally, the acidity of various superacid systems could be determined by UV–vis, NMR, and kinetic measurement. − However, these experiments are usually performed for solution phase, and thus the results depend on the properties of the solvent themselves. That is to say, there is some uncertainty in the solution acidity of a superacid. , In comparison, gas-phase acidity is irrelative with solvent, and thus it could be utilized as a uniform standard to characterize superacid species. , Although there are some examples of which the acidity in gas phase is different from that in solution, , gas-phase acidity has been broadly adopted for the theoretical design of novel superacids. , In principle, the gas-phase acidity could be determined by the changes of the Gibbs free energies of the deprotonation reactions of the systems, Δ G acid .…”