Substituent chemical shifts in NMR. 2—Bromine SCS in rigid molecules

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Abstract
The complete analysis and assignment of the 1H NMR spectra of 2‐exo‐bromonorbornane (1) and 2‐bromoadamantane is reported, using high‐field NMR and COSY and proton—carbon correlation experiments to assign the spectra. These data, together with those for the patent hydrocarbons, when combined with previous analyses of 1‐bromoadamantane, 1‐bromooctadeuterocyclohexane and the simple alkyl bromides, provide a data set of 38 proton SCS values for bromine in molecules of accurately known geometry. The effect of solvent on the SCS was investigated for 1 in five common solvents of very different polarity (C6D12 to DMSO). Only small (< 0.1 ppm) solvent shifts were observed, apart from the 2‐endo and 7s protons. A noteworthy feature of the SCS is the downfield shift of the CHX proton in all the series investigated, as X changes from C1 to Br to I. This is the reverse of the ‘normal’ trend and cannot be explained by present theories of SCS.