A measure of the ionizing power (loosely polarity) of a solvent, based on the maximum wavenumber of the longest wavelength electronic absorption band of: in a given solvent. \(E_{\rm{T}}\), called \(E_{\rm{T}}(30)\) by its originators, is given by: \[E_{\rm{T}} = 2.859\times 10^{-3}\ \nu = 2.859\times 10^{4}\ \lambda ^{-1}\] where \(E_{\rm{T}}\) is in \(\pu{kcal mol-1}\), \(\nu \) is in \(\pu{cm-1}\) and \(\lambda \) is in \(\rm{nm}\). The so-called normalized \(E_{\rm{T}}^{\rm{N}}\) scale is defined as: \[E_{\rm{T}}^{\rm{N}}=\frac{E_{\rm{T}}\left(\rm{solvent}\right)- E_{\rm{T}}\left(\rm{Si}\rm{Me}_{4}\right)}{E_{\rm{T}}\left(\rm{water}\right)- E_{\rm{T}}\left(\rm{Si}\rm{Me}_{4}\right)}=\frac{E_{\rm{T}}\left(\rm{solvent}\right)- 30.7}{32.4}\]