Proton Chemical Shifts and Pi-Electron Distributions in the Hydroxy-Benzenes

Abstract

The proton magnetic resonance spectra of phenol, hydroquinone, catechol, resorcinol, and pyrogallol have been studied, and the chemical shifts of the aromatic protons relative to benzene determined. The chemical shifts for the isolated molecules were obtained by making measurements in two solvents and extrapolating to a medium of unit dielectric constant. The electric fields at the protons from the C–O and O–H bond dipoles were calculated, and their contributions to the proton chemical shifts were removed by employing the approximate relation derived by Buckingham. It was assumed that the remaining parts of the chemical shifts were due to changes in the pi-electron distributions. Hückel molecular orbital calculations were performed, and it was shown that the corrected chemical shifts, δi, can be expressed in the form δi=aqi+a′qi′+a′′qi′′,where qi is the excess number of pi electrons on the ith carbon atom, qi′ is the sum of those on the two adjacent carbon atoms, and qi″ is the sum of those on any oxygen atoms ortho to the ith proton. The constants a, a′, and a″ were evaluated by fitting the chemical-shift data with a least-squares procedure. The derived constants depended sensitively on the values chosen for the necessary Coulomb and resonance integrals. It was not possible to conclude whether sigma inductive effects should be included or not, so it is suggested that they be omitted for the time being. By requiring reasonable values for the constants, i.e., a≅7, a′≅1, and a″≅0 ppm/electron and searching for closest agreement between calculated and observed chemical shifts, it was possible to choose as the most appropriate molecular orbital parameters, αO=αC+0.75 βCC, and βCO=1.0 βCC.