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Type: Artigo de periódico
Title: Orbital interactions and their effects on C-13 NMR chemical shifts for 4,6-disubstituted-2,2-dimethyl-1,3-dioxanes. A theoretical study
Author: Tormena, CF
Dias, LC
Rittner, R
Abstract: A theoretical study is employed to describe the orbital interactions involved in the conformers' stability, the energies for the stereoelectronic interactions, and the corresponding effects of these interactions on the molecular structure (bond lengths) for cis- and trans-4,6-disubstituted-2,2-dimethyl-1,3-dioxanes. For cis-4,6-disubstituted-2,2-dimethyl-1,3-dioxanes, two LPO->sigma*(C(2)-Me(8))(2)-Me(B) interactions are extremely important and the energies involved in these interactions are in the range 6.81-7.58 kcal mol(-1) for the LPO(1)->sigma*(C(2)-Me(8)) and 7.58-7.71 kcal mol(-1) for the LPO(3)->sigma*(C(2)-Me(8)) interaction. These two LPO->sigma*(C(2)-Me(8)) interactions cause an upfield shift, indicating an increased shielding (increased electron density) of the ketal carbon C(2) as well as the axial Me-(8) group in the chair conformation. These LPO-sigma*(C(2)-Me(8)) hyperconjugative anomeric type interactions can explain the C-13 NMR chemical shifts at 19 ppm for the axial methyl group "Me-(8)" and 98.5 ppm for the ketal carbon "C-(2)". The observed results for the trans derivatives showed that for compounds 2a-c (R = -CN, -C&3bond; CH, and -CHO, respectively) the chair conformation is predominant, whereas for 2d,f-h [-CH3, -Ph, -C6H4(p-NO2), -C6H4(p-OCH3) respectively] the twist-boat is the most stable compound and for 2e [-C(CH3)(3)] is the only form.
Country: EUA
Editor: Amer Chemical Soc
Citation: Journal Of Physical Chemistry A. Amer Chemical Soc, v. 109, n. 27, n. 6077, n. 6082, 2005.
Rights: fechado
Identifier DOI: 10.1021/jp052048a
Date Issue: 2005
Appears in Collections:Unicamp - Artigos e Outros Documentos

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