C-13-NMR spectroscopy of beta-nitrostyrenes. II. Mono-, bi- and tri-methoxy phenyl-substitutions and long distance electronic effects

By means of C-13-NMR spectroscopy and AM1 molecular orbital calculations of mono, bi- and tri-methoxy-beta-nitrostyrenes at the meta and para position s, we have characterized a long distance electronic charge transfer pattern on the ethylenic bridge (CH=CH) and on the aromatic ring (Ph) carbon cente rs, determined by the electron-donor nature of the methoxy-substituent grou ps. After a complete spectral assignment of the C-13-NMR signals, we have found a functional dependence of the chemical shifts on the C-1 and C-beta cente rs respect to the C-4 and C-3 methoxy substitution sites on the aromatic ri ng, while in the same molecular series Ca-chemical shifts are practically c onstants. On the other hand, the ISC-NMR chemical shifts of the C-3 and C-4 centers plus the analysis of the AM1 electronic charge density have permit ted us determine the long distance charge transfer effect induced by the C- 4 methoxy substitutions as well as the attenuation of this effect due to th e C-3 methoxy substitutions.