Substituent chemical shifts (SCS) in NMR. Part 5. Mono- and di-fluoro SCS in rigid molecules

Abstract

The complete assignment of the proton, carbon and fluorine NMR spectra of fluorocyclohexane (axial and equatorial conformers), 4-methyl-1,1-difluorocyclohexane, 4-tert-butyl-1,1-difluorocyclohexane, 3-methyl-1,1-difluorocyclohexane and 2,2-difluoronorbornane is reported and the proton substituent chemical shifts obtained. The fluorocyclohexane substituent chemical shifts (SCSs) are in close agreement with monofluoro SCS data obtained from steroids, the SCS of the 2_ax- and 2_eq-H being independent of the orientation of the fluorine. The SCS obtained from fluorocyclohexane are not applicable to the difluorocyclohexane systems and this non-additivity is shown to be general for CF₂ and CF₃ groups. The proton chemical shift calculation scheme previously given for hydrocarbons can now be extended to include fluoroalkanes using the data presented here. It is shown that the proton chemical shifts of a variety of fluoroalkanes can be well predicted on this scheme.