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|Type:||Artigo de periódico|
|Title:||Conformational And Electronic Interaction Studies Of á-substituted Carbonyl Compounds. Part 9. ω-hetero-substituted Acetophenones|
|Abstract:||νCO frequencies and intensities, n→π* CO transition energies and α-methylene carbon chemical shifts were measured for some ω-hetero-substituted acetophenones (φCOCH 2X: X = F, OMe, NMe2, NEt2, Cl, Br, SEt, and I), and their conformations were estimated with the help of molecular-mechanics calculations. The stability of the gauche rotamers is discussed in terms of hyperconjugative interactions between σCX and π*CO orbitals. The carbonyl frequency shifts of the cis rotamers are interpreted as due to the substituent field effect, while the corresponding shifts for the gauche rotamers have been ascribed to an interplay of the inductive and hyperconjugative effects. The abnormal negative νCO gauche shifts and the n→π*CO band bathochromic shifts of the ω-fluoro- and ω-methoxy-acetophenones, and the observed shielding effect on the α-methylene carbon atom of the studied acetophenones, are discussed in terms of π*CO/ σCX and π*CO/X1p orbital overlapping. The higher stabilization of the ω-ethylthioacetophenone gauche rotamer, the largest negative νCO gauche shift, and the lower shielding effect for its methylene carbon have been interpreted as due to the simultaneous occurrence of π*CO/πCS and πCO/σ*CS orbital interactions.|
|Appears in Collections:||Unicamp - Artigos e Outros Documentos|
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