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Type: Artigo de periódico
Title: Conformational Analysis And Stereoelectronic Effects In Trans-1,2-dihalocyclohexanes: 1h Nmr And Theoretical Investigation.
Author: Freitas, Matheus P
Rittner, Roberto
Tormena, Cláudio F
Abraham, Raymond J
Abstract: The conformational equilibrium of trans-1,2-difluoro- (1), trans-1,2-dichloro- (2) and trans-1,2-dibromo-cyclohexane (3) was studied through a combined method of NMR, theoretical calculations and solvation theory. The solvent dependence of the 3JH1,H2 NMR coupling constants together with theoretical calculations allow the direct determination of the conformational equilibria without recourse to model compounds. The coupling constants were obtained with the aid of spectrum simulation, since these symmetric molecules present complex coupling systems. The observed couplings, when analysed by solvation theory and utilising DFT geometries (B3LYP/6-311+G**), gave energy values of E(ee) -E(aa) of 0.10, 0.95 and 1.40 kcalmol(-1) in the vapour phase for 1, 2 and 3, respectively, decreasing to -0.63, 0.36 and 0.93 kcalmol(-1) in CCl4 and to -1.91, -0.80 and -0.05 kcalmol(-1) in DMSO solution. The diaxial preference for all compounds is explained by natural bond orbital (NBO) analysis, which shows important hyperconjugative effects in this conformation. The gauche effect for compounds with more electronegative substituents, which are in gauche arrangement in the ee conformation, also plays a relevant role in more polar solvents.
Subject: Cyclohexanes
Hydrocarbons, Halogenated
Magnetic Resonance Spectroscopy
Molecular Conformation
Molecular Structure
Rights: fechado
Identifier DOI: 10.1016/j.saa.2004.07.007
Date Issue: 2005
Appears in Collections:Artigos e Materiais de Revistas Científicas - Unicamp

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