Solvent shifts in nuclear magnetic resonance spectroscopy. Part VIII. Solvent shifts induced by benzene and toluene in methoxybenzenes: a variable-temperature n.m.r. study
Benzene causes upfield shifts of the methoxyl resonances (relative to carbon tetrachloride as solvent) in twenty methoxybenzenes, with the exception of the 2-methoxyl resonance of 1,2,3-trimethoxybenzene, which suffers a downfield shift. In para-substituted anisoles, the tendency is for an electron-withdrawing substituent to increase the upfield solvent shift of the methoxyl resonance, and for an electron-donating substituent to decrease the solvent shift (relative to anisole as reference compound). The solvent shifts do not show a particularly good correlation with the dipole moments of the molecules, but a better correlation with Hammett's σρ values. Variable-temperature studies in toluene solution have indicated that at any given temperature, p-nitroanisole is complexed to a greater extent with toluene than is anisole, which in turn is complexed more than p-NN-dimethylaminoanisole. Thermodynamic parameters (ΔH, ΔG, ΔS) for complex formation of these three compounds with toluene have been calculated on the basis of an assumed 1:1 complex. It is feasible that the dependence of solvent shift of a methoxyl resonance upon the nature of other substituents in the aromatic ring may be useful in elucidating the structure of natural products.