ONE-BOND CC SPIN-SPIN COUPLING-CONSTANTS IN DERIVATIVES OF BENZENE - NONLINEARITY OF (1)J(CC) VS SUBSTITUENT ELECTRONEGATIVITY

A set of one-bond CC coupling constants has been determined for mono- and disubstituted benzenes. Large 1J(CC) values have been found within the benzene rings bearing highly electronegative substituents such as halogens, methoxy and nitro groups and small values for those with electropositive substituents. The total range of 1J(CC) couplings observed in our work is larger than 50 Hz. These large variations of CC spin-spin couplings are interpreted in terms of Fermi-contact contributions and the redistribution of s-electrons within a CC bond under influence of substituents. Contrary to some previous findings the data obtained in the present work indicate that the relationship between 1J(CC) and the substituent electronegativity is not linear.