Use of stable 2,2,6,6-tetramethylpiperidine N-oxyl radical for the measurements of Lewis acidity of solvents

Abstract

The use of the stable 2,2,6,6-tetramethylpiperidine N-oxyl (TMPNO) radical as a standard base made it possible to introduce a new scale of Lewis acidity for solvents. The parameter E_B is defined as the energy (in kJ mol^–1) of n–π* transition in the TMPNO spectrum measured in different solvents; it correlates very well with other scales of Lewis acidity such as E_T, AN, and Z. Interaction of TMPNO with inorganic acids (H⁺ and Li⁺) in water is different as evidenced by changes in the absorption spectra.