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|Type:||Artigo de periódico|
|Title:||Conformational Analysis. Part 27. Nmr, Solvation And Theoretical Investigation Of Conformational Isomerism In Fluoro- And 1,1-difluoro-acetone|
|Abstract:||The solvent and temperature dependence of the 1H and 13C NMR spectra of fluoroacetone (FA), 1,1-difluoroacetone (DFA) and 1,1,1-trifluoroacetone (TFA) are reported and the 4J HF, 1J CF, and 2J CF couplings analysed using ab initia calculations and solvation theory. In FA the energy difference (E cis - E tr) between the cis (F-C-C=O 0°) and trans (F-C-C=O 180°) conformers is 2.2 kcal mol -1 in the vapour, decreasing to 1.0 kcal mol -1 in CCl4 solution and to -0.6 kcal mol -1 in the pure liquid. In DFA the conformational equilibrium is between the less polar cis (H-C-C=O 0°) and a gauche conformation (H-C-C=O 104°). The energy difference (Eg - E cis) is +0.8 kcal mol -1 in the vapour, decreasing to 0.1 kcal mol -1 in CCl 4 solution and to -0.9 kcal mol -1 in the pure liquid. The vapour state energy difference for FA compares well with that calculated (2.8 kcal mol -1 at MP4/6-31G*). DFA calculations at this level gave only one minimum in the potential surface corresponding to the cis form. A minimum for the gauche conformer was only found when solvation was included in the ab initio calculations, or at much larger basis sets (6-311++G**). The conformer couplings obtained show that the 4J HF coupling (F-C-C-CH 3) is proportional to cos 2 θ, where θ is the F-C-C-C dihedral angle. The 1J CF and 2J CF couplings also show a pronounced orientation dependence which could be of particular utility in those cases where other couplings are not present.|
|Appears in Collections:||Unicamp - Artigos e Outros Documentos|
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