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|Type:||Artigo de periódico|
|Title:||An NMR, IR and theoretical investigation of the methyl effect on conformational isomerism in 3-fluoro-3-methyl-2-butanone and 1-fluoro-3,3-dimethyl-2-butanone|
|Abstract:||The solvent dependence of the H-1 and C-13 NMR spectra of 3-fluoro-3-methyl-2-butanone (FMB) and 1-fluoro-3,3-dimethyl-2-butanone (FDMB) was examined and the (4)J(HF), (1)J(CF) and (2)J(CF) couplings are reported. Density functional theory (DFT) at the B3LYP/6-311 ++ G(2df,2p) level with ZPE (zero point energy) corrections was used to obtain the conformer geometries. In both FMB and FDMB, the DFT method gave only two minima for cis (F-C-C=O, 0degrees) and trans (F-C-C=O, 180degrees) rotamers. Assuming the cis and trans forms, the observed couplings in FM B when analysed by solvation theory gave the energy difference E-cis-E-trans of 3.80 kcal mol(-1) (1 kcal = 4.184 kJ) in the vapour phase (cf. the DFT value of 3.21 kcal mol(-1)), decreasing to 2.6 kcal mol(-1) in CCl4 and to 0.27 kcal mol(-1) in DMSO. In FDMB the observed couplings when analysed similarly by solvation theory gave E-cis - E-trans = 1.80 kcal mol(-1) in the vapour phase, decreasing to 0.47 kcal mol(-1) in CCl4 and to - 1.25 kcal mol(-1) in DMSO. The introduction of a methyl group geminal to the fluorine atom shifts the conformational equilibrium towards the trans rotamer, in contrast to no significant effect when the methyl group is introduced at the alpha-carbon further from the fluorine atom. Copyright (C) 2002 John Wiley Sons, Ltd.|
|Editor:||John Wiley & Sons Ltd|
|Appears in Collections:||Artigos e Materiais de Revistas Científicas - Unicamp|
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