NMR SPECTRA AND AROMATIC RING CURRENTS IN CONJUGATED $HYDROCARBONS^{*}$

Abstract

The 60 Mc high resolution NMR spectra of naphthalene, anthracene, pyrene, perylene and triphenylene have been observed and chemical shifts calculated for infinitely dilute solutions. The spectra have been completely analyzed for the first time. In addition, partial information has been obtained for phenanthrene. These chemical shifts permit a more rigorous test of Pople’s theory of aromatic ring currents. The calculated chemical shifts make use of ring currents calculated by the method of $London^{1}$ for each individual ring. The distance dependence of the resulting chemical shifts was calculated using the tables of Johnson and Bovey. While in most cases the calculated chemical shifts are larger than the observed ones, the hydrogen atoms in phenanthrene which are in van der Waals contact have a chemical shift considerably larger than that calculated. This is believed to be a van der Waals shift similar to that recently observed for gases under high pressures. The similarly located protons in perylene have a much smaller chemical shift which may be due to the marked reduction in the degree of van der Waals contact or to a smaller current in the central ring.