Substituent induced Chemical Shifts (SCS) of 1,4-disubstituted benzenes: Correlation analysis

The C-13 NMR Substituent-induced Chemical Shifts (SCS) of aromatic carbons in. 1,4-disubstituted benzenes were correlated with inductive and mesomeric effects, expressed in sigma(I) and sigma(R) (NMR) scales, and with charge alterations (Delta q) provoked by introducing a new substituent (X) at position C-1. 105 different compounds with groups attached to C-1 (X) and C-4 (Y) in, a sequence: NMe2, NH2, OMe, F, Cl, Br, Me, H, CF3, CN, C(O)OEt, C(O)Me, NO2 and CHO were studied. Correlations of sigma(I) and sigma(R) with SCS and with Delta q, and of SCS with Delta q, showed statistical relevance just for C-4. SCS and Delta q were correlated only with sigma(R) for C-2. No correlation was found for C-2 (ipso-carbon), except between Delta q and sigma(R), while for C-3 the occurrence or not of arty correlation, was strongly dependent on, the classes of compounds, fixed by the substituent at C-4 (Y).