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Type: Artigo de periódico
Title: Stereoelectronic and inductive effects on H-1 and C-13 NMR chemical shifts of some cis-1,3-disubstituted cyclohexanes
Author: de Oliveira, PR
Tasic, L
Rocco, SA
Rittner, R
Abstract: This work presents the substituent effects on the H-1 and C-13 NMR chemical shifts in the cis-isomer of 3-Y-cyclohexanols (Y = Cl, Br, I, CH3, N(CH3)(2) and OCH3) and 3-Y-1-methoxycyclohexanes (Y = F, Cl, Br, I, CH3, N(CH3)(2) and OCH3). It was observed that the H-3 chemical shift, due to the substituent alpha-effect, increases with the increase of substituent electronegativity when Y is from the second row of the periodic table of elements, (CH3 <N(CH3)(2) <OCH3 <F) and decreases with the increase of electronegativity when Y is a halogen (Cl < Br < I). The natural bond orbital (NBO) analysis showed that the alpha-effect on the H-3 chemical shift increases with the decrease of n(Y) --> *sigma(C3-H3a) interaction energy. This interaction energy, for the halogenated compounds, decreases with an increase in size of the halogen, and this is a possible reason for the largest measured chemical shift for H-3 of the iodo-derivatives. The beta-effect of the analyzed compounds showed that the chemical shift of hydrogens at C-2 and C-4 increases with the decrease of n(Y) --> *sigma(C2-C3) and n(Y) --> *sigma(C3-C4) interaction energies, respectively, showing a behavior similar to H-3. The alpha-effect on C-13 chemical shifts correlates well with substituent electronegativity, while the beta-effect is inversely related to electronegativity in halogenated compounds. NBO analysis indicated that the substituent inductive effect is the predominant effect on C-13 NMR chemical shift changes for the alpha-carbon. It was also observed that C-2 and C-4 chemical shifts for compounds with N(CH3)(2), OCH3 and F are more shielded in comparison to the compounds having a halogen, most probably because of the larger interaction of the lone pair of more electronegative atoms (n(N) > n(O) > n(F)) with *sigma(C2-C3), *sigma(C3-C4) and *sigma(C3-H3a) in comparison with the same type of interaction with the lone pair of the other halogens. Copyright (C) 2006 John Wiley & Sons, Ltd.
Subject: H-1 NMR
C-13 NMR
stereoelectronic effect
1,3-disubstituted cyclohexanes
chemical shifts
natural bond orbital (NBO) analysis
Country: Inglaterra
Editor: John Wiley & Sons Ltd
Citation: Magnetic Resonance In Chemistry. John Wiley & Sons Ltd, v. 44, n. 8, n. 790, n. 796, 2006.
Rights: fechado
Identifier DOI: 10.1002/mrc.1850
Date Issue: 2006
Appears in Collections:Unicamp - Artigos e Outros Documentos

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