Conformational analysis. Part 33. 1 An NMR, solvation and theoretical investigation of conformational isomerism in N , N -dimethyl fl uoroacetamide and N , N -dimethyl- (cid:1) - fl uoropropionamide

The solvent and temperature dependence of the 1 H and 13 C NMR spectra of N , N -dimethyl fl uoroacetamide (DMFA) and N , N -dimethyl- α - fl uoropropionamide (DMFP) are reported and the 5 J CF , 1 J CF and 4 J CF couplings analysed by solvation theory. Density function theory (DFT) at the B3LYP/6-311 (cid:1) 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–C (cid:2)(cid:2) O, 0 (cid:3) ) and gauche (F–C–C (cid:2)(cid:2) O, 140.6 (cid:3) ) 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 di ff erence ( E cis (cid:4) E g ) of 2.5 kcal mol (cid:4) 1 in the vapour phase, ( cf . the ab initio value of 2.3 kcal mol (cid:4) 1 ) decreasing to 0.87 kcal mol (cid:4) 1 in CCl 4 and to (cid:4) 1.29 kcal mol (cid:4) 1 in DMSO. In DMFP the ab initio calculations gave three minima; the cis (F–C–C (cid:2)(cid:2) O, 30.4 (cid:3) ), gauche- 1 (F–C–C (cid:2)(cid:2) O, 144.7 (cid:3) ) and gauche- 2 (F–C–C (cid:2)(cid:2) O, (cid:4) 124.1 (cid:3) ) rotamers with ( E cis (cid:4) E g 2 ) equal to 2.5 kcal mol (cid:4) 1 and ( E g 1 (cid:4) E g 2 ) equal to 0.3 kcal mol (cid:4) 1 . The observed couplings were analysed by solvation theory assuming one “average” gauche conformer to give ( E cis (cid:4) E g (AV) ) equal to 2.1 kcal mol (cid:4) 1 in the vapour phase, decreasing to 0.83 kcal mol (cid:4) 1 in CCl 4 and to (cid:4) 1.11 kcal mol (cid:4) 1 in DMSO.