An assessment of simple theories of chemical shifts of aromatic protons as applied to substituted naphthalenes

Abstract

The proton chemical shifts of a number of 1-substituted naphthalenes are examined in the light of some of the simple theories of the origins of substituent chemical shifts claimed to be successful for benzene derivatives. It is shown that not one of these methods of predicting proton shifts gives correct answers for all positions in naphthalenes, and it is concluded that attempts to use proton shielding as an indication of molecular structure must be treated with caution. This point is illustrated by a discussion of the data for 1-naphthaldehyde, 2-hydroxy-1-naphthaldehyde, and 2-methyl-1-nitronaphthalene, and it is concluded that reliable structural information can be obtained only when changes in proton shielding can be ascribed to changes in the electron density on the proton.