Chemical Shifts and Pi-Electron Densities in Substituted Benzenes

The theoretical values of the proton, carbon, and fluorine chemical shifts of the substituted benzenes with a variety of substituents have been calculated and compared with the experimentally observed chemical shifts. It is found that the para shieldings, as assumed, are primarily determined by the pi-electron density variations induced by the substituents. However, at ortho and meta positions, factors other than pi-electron densities become important in contributing to the observed shieldings. In disubstituted benzenes, it appears that the proposed additivity relationship of the substituent effects may be a combined effect of at least two separate additive mechanisms, one of which is from the pi-electron variation; both, however, contribute to the shieldings to an approximately equal extent.