13C nuclear magnetic resonance studies of some fluorinated and trifluoromethylated aromatic compounds. Studies on13C–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.