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Methoxy group conformation effects on 13 C NMR parameters in 1‐ cis ‐methoxy‐ and 1‐ trans ‐methoxy‐1,3‐ trans ‐butadiene
Author(s) -
Esteban Angel L.,
Galache Maria P.,
Diez Ernesto,
Biekofsky Rodolfo R.,
Contreras Rubén H.
Publication year - 1994
Publication title -
magnetic resonance in chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.483
H-Index - 72
eISSN - 1097-458X
pISSN - 0749-1581
DOI - 10.1002/mrc.1260320402
Subject(s) - chemistry , ab initio , carbon 13 nmr , chemical shift , cis–trans isomerism , nmr spectra database , crystallography , proton nmr , polarizability , proton , stereochemistry , molecular orbital , spectral line , computational chemistry , molecule , organic chemistry , physics , astronomy , quantum mechanics
The proton‐coupled 13 C NMR spectra of 1‐ trans ‐methoxy‐ (2) and 1‐ cis ‐methoxy‐1,3‐ trans ‐butadiene (3) are consistent with a methoxy heavy atom planar conformation with s‐syn and s‐anti orientations, respectively. Ab initio 6–31G* calculations confirmed such conformations. They were taken as model compounds to determine the influence of electrostatic interactions on the methoxy 13 C NMR parameters. A shielding increase of 4 ppm is observed in 2, with respect to 3, for the OMe 13 C chemical shift in an s‐syn conformation and is ascribed to the attraction between the methyl and vinyl moieties as proposed by Li and Chesnut. The methyl 1 J (C,H) coupling is not affected by this interaction, showing that the carbon 2p electrons are more polarizable than those in the 2s orbital.