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. $E_{\text{T}}$, called $E_{\text{T}}(30)$ by its originators, is given by: $$E_{\text{T}}=2.859\times {10}^{-3}\nu =2.859\times {10}^4{\lambda }^{-1}$$ where $E_{\text{T}}$ is in $\mathrm{kcal}{\mathrm{mol}}^{-1}$, $\nu$ is in ${\text{cm}}^{-1}$ and $\lambda$ is in $\text{nm}$. The so-called normalized $E_{\text{T}}^{\text{N}}$ scale is defined as: $$E_{\text{T}}^{\text{N}}=\frac{E_{\text{T}}\left(\text{solvent}\right)-E_{\text{T}}\left(\text{Si}{\text{Me}}_4\right)}{E_{\text{T}}\left(\text{water}\right)-E_{\text{T}}\left(\text{Si}{\text{Me}}_4\right)}=\frac{E_{\text{T}}\left(\text{solvent}\right)-30.7}{32.4}$$