Conformational analysis. Part 33.1 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 5JCF, 1JCF and 4JCF 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 (Ecis − Eg) 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 (Ecis − Eg2) equal to 2.5 kcal mol−1 and (Eg1 − Eg2) equal to 0.3 kcal mol−1. The observed couplings were analysed by solvation theory assuming one “average” gauche conformer to give (Ecis − Eg(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.