Conformational analysis. Part 22. An NMR and theoretical investigation of the gauche effect in fluoroethanols

Abstract

The ABCX NMR spectrum of the C·CH(OH)·CH₂F fragment of 1-(4-bromophenyl)-2-fluoroethanol (BPFE) in eight solvents of different polarities was recorded and fully analysed. ¹³C and NOE measurements confirmed the assignment of the prochiral methylene protons. The AA′BB′XX′R NMR spectrum of 1,3-difluoropropan-2-ol (DFP) was completely analysed in four solvents of different polarities. The couplings showed little solvent dependence except for D₂O solution.

From these analyses the conformer populations and hence relative energies of the possible conformers of these molecules were obtained. In BPFE, the gt rotamer in which fluorine is gauche to the hydroxy and trans(anti) to the phenyl group is favoured in all solvents but the rotamer energies are very dependent on the solvent. ΔE(gg–gt) varies from 1.6 kcal mol^–1 in CCl₄ to 0.5 kcal mol ^–1 in DMSO, and ΔE(tg–gt) varies from 1.0 to 0.5 kcal mol^–1 in the same solvents. In DFP three populated conformers were found, two with a trans–gauche arrangement of the fluorine atoms and one with a gauche–gauche arrangement.

Ab initio MO calculations at the 6-31G level correctly predict the conformer energies except that the gauche vs. trans F ⋯ O orientation is calculated to be 1–2 kcal mol^–1 less stable than observed. Other theoretical studies show that including polarisation functions does not affect this general result. It is suggested that C–F bond shortening in the trans orientation may contribute to the higher energy of this form and this would also explain the fact that the gauche effect is not observed for the comparable CHF₂ group.