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A critical analysis is given of currently popular scales of solvent Lewis basicity (Koppel–Palm, Paju–Koppel, and Gutmann) with particular consideration to and extension of the Kamlet–Taft scale, BKT. Measurements for 22 new solvents were carried out and BKT values for 70 solvents are reported. A rough agreement between BKT and other parameters for solvent basicity has been found. A linear dependence of the Koppel–Palm E values with ET was shown for solvents of higher polarity (ET > 42).

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 EB 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 ET, AN, and Z. Interaction of TMPNO with inorganic acids (H+ and Li+) in water is different as evidenced by changes in the absorption spectra.