Solvent effects on the boron–fluorine coupling constant and on fluorine exchange in the tetrafluoroborate anion

Abstract

The 11B—19F spin–spin coupling constant in AgBF4 is shown to have an opposite sign in water from that found in a number of organic solvents. Application of the hypothesis of Bell and Danyluk indicates that it has a positive sign in water. Evidence is presented for association, probably in the form of solvent-separated ion pairs, in aqueous LiBF4 and NaBF4 solutions, and for the formation of more tightly bound ion pairs in solvents of lower dielectric constant. In contrast, there apparently is no solute association in AgBF4 solutions in water, DMSO, DMF, or acetonitrile. The pronounced changes observed in the 11B—19F coupling constant support the postulate that its small size is a result of near cancellation of large opposing terms.The collapse of the 10B—11B isotope shift at high temperatures in the 19F spectra of solutions of AgBF4 in acetonitrile and in acetone provides evidence for exchange of fluorine among boron atoms; a mechanism is proposed for this process.

Nuclear magnetic resonance studies of ketone•BF3 complexes. II. The boron trifluoride catalyzed condensation of acetone

“…Such samples, which could be stored at -78" for a month with no decomposition, gave low-temperature (-80") spectra very similar to those obtained in methylene chloride solutions of acetone and BF, with acetone present in excess (2). The only large absorption, which arises from acetone.BF,, was split by a 1°B -llB isotope shift into a doublet of relative areas 1 :4 and relative chemical shift 0.066 p.p.m.…”

“…studies (2) have shown that only the 1 :I complex forms, and that in the presence of excess ketone the amount of non-complexed BF, present is negligible and should not have any effect on the n.m.r. spectrum; thus we might expect a single 19F resonance arising from the acetone.BF, complex.…”

“…study of two distinct rapid reversible reactions, namely complexes in methylene chloride solution (2), breaking and re-forming of the donor~acceptor we observed that even after heating dilute solubond and fluorine exchange among boron atoms tions the at loo" for (see part I of this series, ref. 2).…”

“…study of two distinct rapid reversible reactions, namely complexes in methylene chloride solution (2), breaking and re-forming of the donor~acceptor we observed that even after heating dilute solubond and fluorine exchange among boron atoms tions the at loo" for (see part I of this series, ref. 2). There have been short periods there was very little color change in conflicting reports concerning the stability of the the solution; the absence of extensive reaction ketone.BF, complexes.…”

Bis- and tris(amidine)fluoroboron cations and mixed tetrahaloborate anions: NMR studies of mixed boron trihalide adduct redistribution reactions involving amidines as strong nitrogen bases

“…Since the magnetogyric ratio of 'OB is only 113 that of "B, the ma nitude of J("B-'~F) is on1 113 that of J("B-'~F) f B (27). The ' F isotope shift between ' BFC1, and "BFC~, is 0.092 ppm, appreciably greater than the corresponding value for BF,-, 0.050 ppm (28), ketone.BF3 adducts, 0.066 ppm (29), or (quinuclidine),BF,+, 0.064 ppm, in accord with heavier nearby atoms increasing the effect (30).…”