Experimental determination of the conformational free energies (A values) of fluorinated substituents in cyclohexane by dynamic 19F NMR spectroscopy. Part 1. Description of the method for the trifluoromethyl group

Abstract

The conformational free energy (A value) of the trifluoromethyl group was determined by variable temperature 19F NMR studies of trifluoromethylcyclohexanes bearing a substituent at the 4 position. 19F NMR chemical shifts for each conformer above the coalescence temperature were obtained by extrapolation from low temperature values, allowing the high precision determination of the equilibrium constants, and then the thermodynamic parameters (ΔG°, ΔH°, ΔS°). The validity of the hypothesis that substituent parameters are additive was discussed. Thermodynamic data of phenyl and cyclohexyl groups were also given.