Dynamic nuclear magnetic resonance. An alternative method for acidity measurements. Application to superacids

Abstract

The changes in the nuclear configurations of reactants associated with the donation and acceptance of electrons were discussed previously by the present authors. See K. Fukui and H. Fujimoto, Bull. Chem. Soc. Jpn., 42, 3399 (1969). Equation 2indicates that donor MO loses electrons more rapidly than acceptor MO receives. Since the weakening of bonds between reaction sites and adjacent atoms in either reactant upon acceptance of electrons into unoccupied MO's and/or donation of electrons from occupied MO's gets smaller as the conjugation chain becomes longer, the larger conjugated system would preferably play the donor part in order to bring about harmonious displacements of nuclei in both of the reactants along a reaction path. (10) This conclusion is derived by the application of the Hellmann-Feynman theorem to chemically interacting systems (H. Hellmann, "Einführung in

Superacids

“…The acidity function of HSO3F increases from -15.6 to -21.0 on addition of 25 mole percent SbF5 (19,20), as shown in value for HF-SbF5 is already -21.0, a thousand times stronger than the value for fluorosulfuric acid with the same SbF5 concentration. At present it is difficult to estimate the acidity of 1: 1 HF-SbF5, but a value of -28 can be predicted (20) on the basis of isomerization kinetics data.…”

“…Spectroscopic techniques such as 'H and 13C NMR and infrared, ultraviolet, and electron spectroscopy have been employed to characterize carbocations (10). Typical alkyl and cycloalkyl cations gener- 19 17 0 I 15 13 H503 F-SbFsl5 HF-SbF5 lo ated and studied in these superacidic media are shown in Fig. 3, and the 'H NMR spectra of 3 and 4 as well as the tert-amyl cation and the tricyclopropylmethyl cation are shown in Fig.…”

Superelectrophiles: Charge–Charge Repulsive Effects

“…Using dynamic NMR techniques, a charge-charge repulsive interaction was shown to influence the rotational barrier in a superelectrophile ( Figure 7). [10] In FSO 3 H (H o À15), p-anisaldehyde forms the carboxonium ion 34 and the rotational barrier was estimated to be 18.6 kcal mol…”

General Aspects

“…45 By combining this method with the previously discussed chemical shift method, which is sensitive in the 0.05-20 range for the ionization ratio, the acidity could be measured over more than 5 H 0 units with the same indicator. Because of a large difference in activation energies between the rotation barriers in the monoand diprotonated aldehyde, the observed rates are very sensitive to the concentration of BH þ .…”