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|Type:||Artigo de periódico|
|Title:||Conformational analysis. Part 33. An NMR, solvation and theoretical investigation of conformational isomerism in N,N-dimethylfluoroacetamide and N,N-dimethyl-alpha-fluoropropionamide|
|Abstract:||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 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-C=O, 0 degrees) and gauche (F-C-C=O, 140.6 degrees) 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-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 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.|
|Editor:||Royal Soc Chemistry|
|Appears in Collections:||Unicamp - Artigos e Outros Documentos|
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