Theoretical study on the influence of different para-substituents on 13C NMR of the single carbonyl curcumin analogues

The structure of eight kinds of different para-substituents curcumin analogues has been optimized at the level of B3LYP/6-31G(d,p), under which the stability has been verified by means of vibration analysis. Moreover, NMR spectra of curcumin analogues compounds have been studied at the level of B3LYP/6-311G(d,p) by GIAO method. The results show that the structure of eight compounds, a larger conjugated system, has good planarity. The effect of ortho-substituents on bond lengths and bond angles is greater than para and meta. Different substituents and different positions of substituents all have different influence on NMR of the single carbonyl curcumin analogues. In general, after the hydrogen atom on the benzene ring is substituted by other groups, the δ value of α-C changes significantly, the δ value of ortho-carbon atom may also have great change, but the δ value change of meta-carbon atoms is not too obvious. The effect of substituent electronegativity on α-C atoms presents obvious regularity, while the influence of conjugate effect on carbon atoms of benzene ring is more complex. Finally, the bigger substituted alkyl is, the more the δ value of α-C increases.