Methyl Substitution Effects on the Proton Chemical Shifts in Benzene

Methyl substitution effects on aromatic and methyl proton chemical shifts in several mono-, di-, and trimethyl benzenes are studied. A new method for obtaining the changes in the ring proton chemical shifts from those of methyl proton shifts at the corresponding positions is used. The extra jr-electron densities in toluene are calculated using the already known relation between the jr-electron densities and the proton shifts in aromatic systems. An inverse relationship is obtained between the ionization potentials and the total methyl effects on the chemical shifts in this series of compounds as one would expect. Dipole moment of toluene is calculated, and a reasonably good agreement is found between the experimentally observed and the calculated dipole moment.