Scope and limitation of the inductive effect : importance of charged substituents

The interaction of two relatively distant charged or dipolar groups was evaluated on 1,4‐disubstituted bicyclo[2.2.2]octane derivatives as the reaction energy of the isodesmic–homodesmotic reaction, in which this derivative is formed from two mono derivatives. Energies of the individual compounds were calculated at the B3LYP/6–311+G(d,p) level. All combinations of 12 common substituents were investigated, representing 78 reaction energies in a 12×12 data matrix. The behaviour of charged and dipolar substituents is considerably different. Interaction with a charged group is described precisely ( R >0.997) by one parameter, essentially identical with the standard inductive constant σ I . Interaction of two dipolar substituents depends also on σ I , but requires an additional parameter, important particularly in the case of donor substituents (NH 2 , OH, Cl), less important to insignificant with the others. It is significantly correlated with the electronegativity of the first atom. Alternatively, it can be evaluated as a new parameter characterizing the substituent effect, say a new scale of group electronegativity different from the suggested scales. With two parameters, the interaction of polar substituents is expressed with similar high precision ( R >0.99) as with charged substituents and one parameter. Copyright © 2004 John Wiley & Sons, Ltd.