Yukawa–Tsuno equation

A multiparameter extension of the Hammett equation to quantify the role of enhanced resonance effects on the reactivity of meta- and para-substituted benzene derivatives, e.g.

$log 10 k = log 10 k 0 + ρ [ σ + r ( σ + − σ ) ]$

The parameter $r$ gives the enhanced resonance effect on the scale ( $σ + – σ$ ) or ( $σ – σ$ ), respectively.