13C nuclear magnetic resonance studies of some fluorinated and trifluoromethylated aromatic compounds. Studies on 13C–19F coupling constants

Abstract

Carbon-13 n.m.r. spectra of a wide variety of carbocyclic and heterocyclic fluoro-, difluoro-, trifluoromethyl-, and fluoro-trifluoromethyl-substituted aromatic compounds have been examined and assigned. Substituent effects on the ¹³C chemical shifts and one-bond and long range ¹³C–¹⁹F scalar coupling constants have been extracted. Experimental coupling constant data are compared with representative calculated results using INDO semi-empirical molecular orbital theory. The poorest agreement is for geminal and long range ¹³C–¹⁹F coupling, which can only be partially attributable to the neglect of orbital and spin dipolar mechanisms. By means of a valence-bond bond-order formulation for coupling constants, an expression is derived for the relationship between ¹³C–¹⁹F and ¹H–¹⁹F coupling constants. The previously used proportionality is shown to be an over-simplification. Systematic trends of the great variety of experimental ¹³C–¹⁹F coupling constant data with structural and electronic factors are discussed qualitatively. The substituent effect of the trifluoromethyl group on the ¹³C chemical shifts in benzene and naphthalene is discussed and it is proposed that field effects are important in explaining the chemical shift trends.