Long distance electronic effects of para-substituted β-nitrostyrenes by 13C-NMR spectroscopy

Abstract

By means of 13C-NMR spectroscopy and AM1 molecular orbital calculations of para-β-nitrostyrenes, we have found a characteristic long distance charge transfer pattern, where the olefinic bridge (CHCH) and the aromatic ring (Ph) carbon centres are perturbed according to the donor-nature of the para-substituent groups. After a complete spectral assignment of the 13C-NMR signals, the chemical shifts ( δ) of the C 1, C 3 and C β centres show a linear functional dependence with the charge densities ( q AM1), while in the same molecular series C 2 and C α are practically constants. On the other hand, an analysis of the electron-donor substituent effect at the para-position of the aromatic carbonyl compounds on the C 4 centre, has permitted us to find a good correlation between the C 4 chemical shift and the electronegativity of this vicinal centre.