The shift △v of the n→π^* absorption maxima in saturated aliphatic carbonyl compounds is correlated by the empirical equation
△v=10⁴∑[σ₁-σ_R(σ_R⁺] (in wave numbers)
where σ₁ and σ_R are Taft's inductive and resonance parameters respectively.σ_R⁺, which is equal to σ_P⁺-σ₁,is used when the substituent is direct polar conjugated with the carbonyl group, and σ_P⁺ is Brown's electrophilic substituent constant.