Taft equation in the light of NBO computations : Introduction of a novel polar computational substituent constant scale 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 6 H 11 [R = Methyl, Ethyl, n -Propyl, Isopropyl, n -Butyl, Isobutyl, sec -Butyl, tert -Butyl, Neopentyl, CH(CH 2 CH 3 ) 2 , CH(CH 3 )C(CH 3 ) 3 , C(CH 3 ) 2 CH 2 CH 3 , C(CH 3 ) 2 C(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.