dual substituent-parameter equation

https://doi.org/10.1351/goldbook.D01866
In a general sense, any equation which expresses substituent effects in terms of two parameters. However, in practice the term is used more specifically for an equation for summarizing the effects of meta- or para- substituents (i = m or p) X on chemical reactivity, spectroscopic properties, etc. of a probe site Y in benzene or other aromatic system. Pi=ρIi σI+ρRi σR P is the magnitude of the property Y for substituent X, expressed relative to the property for X = H; σI and σR are inductive or polar and
resonance
substituent constants, respectively, there being various scales for σR;ρI and ρR are the corresponding regression coefficients.
See also:
extended Hammett equation
Source:
PAC, 1994, 66, 1077. (Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)) on page 1107 [Terms] [Paper]