Proton Magnetic Resonance of Dimethyl Phthalates

Abstract

We have studied the high-resolution proton magnetic resonance spectra of the ring protons in dimethyl phthalate, dimethyl isophthalate, and dimethyl terephthalate. The proton chemical shifts were determined relative to benzene in a series of solvents of widely varying dielectric constant, with both species at infinite dilution. This effectively removes the solvent van der Waals contribution to the observed shifts, and the remaining shifts are in accord with Buckingham's theory of dielectric reaction fields. Extrapolation to a medium of unit dielectric constant was performed in order to obtain chemical shifts typical of isolated molecules. These values were then approximately corrected for intramolecular electric and magnetic fields. For the two nearly planar molecules, the corrected values were correlated with pi-electronic charge densities obtained by Hückel molecular-orbital calculations. The failure of this correlation in the case of dimethyl phthalate can be attributed to the large deviations from planarity due to extreme steric hindrance.