Modeling of substituent effects: self-parametrization, a case of interdependence of the coefficients, illustrated on the linear plus cross-term models
Résumé
The effect of one or several substituents X-i on a molecular property f (reaction rate, spectroscopic property, etc.) is often described in terms of substituent parameters lambda(i) (X) depending on the nature X of the substituent, and the position i of substitution. We call self-parametrization the case when the substituents are parameterized by the value lambda(i) (X) = f(i) (X) of f itself in the molecule monosubstituted by X in position i. On the example of the linear plus cross-term models, we show that self-parametrization implies that the coefficients of the model are interdependent. The theoretical relations between them are given, for equivalent or nonequivalent positions of substitution, and tested on measured acid-base equilibrium constants, and NMR coupling constants.