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

Abstract

The validity of the Taft equation: log( k R/ k CH3) = ρ ∗ σ ∗ + δE S was studied with the aid of NBO computational results concerning cyclohexyl esters RCOOC 6H 11 [R = Methyl, Ethyl, n-Propyl, Isopropyl, n-Butyl, Isobutyl, sec-Butyl, tert-Butyl, Neopentyl, CH(CH 2CH 3) 2, CH(CH 3)C(CH 3) 3, C(CH 3) 2CH 2CH 3, C(CH 3) 2C(CH 3) 3, CH(CH 3)(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.