Taft equation in the light of NBO computations. Introduction of a novel polar computational substituent constant scale for alkyl groups

The validity of the Taft equation: log(kR/kCH3) = ρ∗σ∗ + δES was studied with the aid of NBO computational results concerning cyclohexyl esters RCOOC6H11 [R = Methyl, Ethyl, n-Propyl, Isopropyl, n-Butyl, Isobutyl, sec-Butyl, tert-Butyl, Neopentyl, CH(CH2CH3)2, CH(CH3)C(CH3)3, C(CH3)2CH2CH3, C(CH3)2C(CH3)3, CH(CH3)(Np), CH(iPr)(tBu), C(Me)(Et)(iPr), C(Et)2(tBu) or C(Et)(iPr)(tBu)]. It was proved that the σ∗(alkyl) value is a composite substitutent constant including the polar and steric contributions. A novel computational σ q ∗ substituent constant scale is presented based on the NBO atomic charges of the α-carbon and the computational total steric exchange energies E(ster) of the cyclohexyl esters specified above. The method used offers a useful way to calculate σ∗(alkyl) values for alkyl groups for which experimental Taft’s polar σ∗ parameters are not available.