Abstraet
-i3C study of protonated para, para' disubstituted chalcones X-C,H,-CH=CH-C(OH)+-C,H,-Y, shows for the chemical shift of the carbenium center important deviations from a strict additivity of Substituent Induced Shifts observed in the corresponding monosubstituted ions. By collecting the experimental data into sub-populations where only one substituent (X or Y) varies-the other remaining fixed-a network of linear homogeneous correlations (6/S) is obtained. Their largely variable slopes indicate that the susceptibility of the carbenium site to the perturbations induced by one of the substituent groups depends on the nature of the other substituent group. The definition of 'Intrinsic Substituent Parameters' (derived from the SCS observed, in these ions, on the aromatic para positions) allows to separate the action of a substituent group on the electronic interaction mechanism and the global perturbation detected on the observation site.
For a quantitative evaluation of these interactions, a Factorization Model is proposed, where deviations to additivity of the SCS are expressed as a product of the intrinsic parameters of X and Y groups. This model is successfully applied to r3C spectra of protonated chalcones and benzophenones.
The parallelism between these intrinsic group effects and the SCS observed in monosubstituted benzenes points out the prime importance in these cationic systems, of rr polarisation effects modulating the electron transfer toward the carbenium center within a concerted n-inductive mesomeric action.