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

The solvent and temperature dependence of the H-1 and C-13 NMR spectra of N ,N-dimethylfluoroacetamide (DMFA) and N,N-dimethyl-alpha-fluoropropionamide (DMFP) are reported and the (5)J(CF), (1)J(CF) and (4)J(CF) couplings anal ysed by solvation theory. Density function theory (DFT) at the B3LYP/6-311G(d,p) level with ZPE (zero point energy) corrections was used to obtain th e conformer geometries. In DMFA, the DFT method gave only two minima for th e cis (F-C-C=O, 0 degrees) and gauche (F-C-C=O, 140.6 degrees) rotamers. Th e trans rotamer was not a minimum in the energy surface. Assuming only the cis and gauche forms, the observed couplings when analysed by solvation the ory gave the energy difference (E-cis - E-g) of 2.5 kcal mol(-1) in the vap our 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 ini tio calculations gave three minima; the cis (F-C-C=O, 30.4 degrees), gauche -1 (F-C-C=O, 144.7 degrees) and gauche-2 (F-C-C=O, -124.1 degrees) 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 a ssuming 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 CCl 4 and to -1.11 kcal mol(-1) in DMSO.