Protonation of p-methoxybenzhydryl cations; a new series of indicators for superacid acidity measurements by nuclear magnetic resonance spectroscopy

Abstract

The behaviour of (p-substituted phenyl)(p-methoxyphenyl)methanol derivatives dissolved in superacid mixtures (HF, HFSO₃, or CF₃SO₃H with various proportions of SbF₅) has been studied by n.m.r. spectroscopy. Increasing the SbF₅ concentration causes drastic changes in Chemical shifts due to stable O-protonation of the methoxy group in the highest acidity media. By using these bases as indicators for superacid acidity measurements we have shown that the acidity of these three systems does not increase as fast as previously claimed, and also that the acidity of the CF₃SO₃H–SbF₅ system remains lower than that of the HFSO₃–SbF₅ system in the 0–50% SbF₅ range.