Substituent effects in aromatic proton nuclear magnetic resonance spectra. Part VI. [2H6]benzene-induced solvent shifts in 4,4′-disubstituted 2,2′,6,6′-tetramethylbiphenyls and related compounds

Abstract

Twelve 2,2′,6,6′-tetramethylbiphenyls 4,4′-disubstituted with NH₂, NMe₂, OH, OMe, Cl, Br, I, CO₂H, CO₂Me, CN, NO₂, and H, were synthesized and the [²H₆]benzene-induced solvent shifts Δ{=δ(carbon tetrachloride)–δ[²H₆]benzene} of their ¹H n.m.r. spectra were compared with those of the corresponding 1-substituted 3,5-di-methylbenzenes. The Δ values are subdivided into two contributions, one due to benzene solvent molecule co-ordinated with the substituent, and the other due to benzene solvent molecule(s) weakly associated with the rest of the molecule. It was assumed that the Δ values of unsubstituted hydrocarbons represent the latter contribution, while the difference in Δ between substituted and unsubstituted species (substituent solvent shift) corresponds to the former contribution. The solvent shifts in 2,4′-disubstituted 2′,4,6,6′-tetramethylbiphenyls (IV) are also consistent with this picture.