Dimroth–Reichardt ET parameter

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. ET, called ET(30) by its originators, is given by: \[E_{\text{T}} = 2.859\times 10^{-3}\ \nu = 2.859\times 10^{4}\ \lambda ^{-1}\] where ET is in kcal mol-1, ν is in cm-1 and λ is in nm. The so-called normalized ETN scale is defined as: \[E_{\text{T}}^{\text{N}}=\frac{E_{\text{T}}\left(\text{solvent}\right)- E_{\text{T}}\left(\text{Si}\text{Me}_{4}\right)}{E_{\text{T}}\left(\text{water}\right)- E_{\text{T}}\left(\text{Si}\text{Me}_{4}\right)}=\frac{E_{\text{T}}\left(\text{solvent}\right)- 30.7}{32.4}\]
See also: Grunwald–Winstein equation, Z-value
PAC, 1994, 66, 1077. 'Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)' on page 1106 (https://doi.org/10.1351/pac199466051077)