13C-NMR spectroscopy of para-substituted benzylideneacetones. I. Long distance electronic effects

Abstract

Para-benzylideneacetones present a characteristic long distance charge transfer pattern, where the olefinic bridge (CH=CH) and the aromatic ring (Ph) carbon centers are perturbed according to the nature of the para-substituent groups. By means of 13C-NMR spectroscopy and AM1 molecular orbital calculations we have found that in this molecular series the chemical shifts (δ) and the charge densities (qAM1) corresponding to the C3, C1 and Cβ centers follow a functional dependence of the type: δ = a qAM1 + δ°, while C2, Cα and Cco are practically constants. On the other hand, after a complete spectral assignment of the 13C-NMR signals, an analysis of the electron-donor substituent effect at the para-position of the aromatic carbonyl compounds on the C4 center, has permitted us to find a good correlation between the C4 spectral shift and the electronegativity of this vicinal center.