PROTON MAGNETIC RESONANCE SHIFTS AND THE ELECTRON DENSITY DISTRIBUTION IN AROMATIC SYSTEMS

Abstract

The quantitative nature of the empirical linear correlation between the proton resonance shifts in aromatic molecules and the local electron density on the carbon atom to which the proton is bonded has been investigated. The application of this relationship to the determination of the electron density distribution of a variety of aromatic molecules, as well as the main limitations inherent in this application, are discussed. Electron density distributions have been derived for aniline, anisole, azulene, acepleiadylene, pyridine, pyridinium ion, and for the aromatic ions pentalenyl, indenyl, fluorenyl, triphenylmethyl cation, and azulene dianion, and are compared with those calculated by molecular orbital methods. In general there is a fairly good correspondence between the experimental and calculated density values, although for the aromatic ions the excess charge tends to be somewhat more uniformly distributed over the molecule than would be indicated by simple MO calculations.