Conformational analysis. Part 33. An NMR, solvation and theoretical investigation of conformational isomerism in N,N-dimethylfluoroacetamide and N,N-dimethyl-α-fluoropropionamide

Abstract

The solvent and temperature dependence of the ¹H and ¹³C NMR spectra of N,N-dimethylfluoroacetamide (DMFA) and N,N-dimethyl-α-fluoropropionamide (DMFP) are reported and the ⁵J_CF, ¹J_CF and ⁴J_CF couplings analysed by solvation theory. Density function theory (DFT) at the B3LYP/6-311+G(d,p) level with ZPE (zero point energy) corrections was used to obtain the conformer geometries. In DMFA, the DFT method gave only two minima for the cis (F–C–CO, 0°) and gauche (F–C–CO, 140.6°) rotamers. The trans rotamer was not a minimum in the energy surface. Assuming only the cis and gauche forms, the observed couplings when analysed by solvation theory gave the energy difference (E_cis − E_g) of 2.5 kcal mol⁻¹ in the vapour phase, (cf. the ab initio value of 2.3 kcal mol⁻¹) decreasing to 0.87 kcal mol⁻¹ in CCl₄ and to −1.29 kcal mol⁻¹ in DMSO. In DMFP the ab initio calculations gave three minima; the cis (F–C–CO, 30.4°), gauche-1 (F–C–CO, 144.7°) and gauche-2 (F–C–CO, −124.1°) rotamers with (E_cis − E_g2) equal to 2.5 kcal mol⁻¹ and (E_g1 − E_g2) equal to 0.3 kcal mol⁻¹. The observed couplings were analysed by solvation theory assuming one “average” gauche conformer to give (E_cis − E_g(AV)) equal to 2.1 kcal mol⁻¹ in the vapour phase, decreasing to 0.83 kcal mol⁻¹ in CCl₄ and to −1.11 kcal mol⁻¹ in DMSO.