Substituent Effects on 13C NMR and 1H NMR Chemical Shifts of CH=N in Multi-substituted Benzylideneanilines

Abstract

Fifty-three samples of multi-substituted benzylideneanilines XArCH=NArYs (abbreviated XBAYs) were synthesized and their NMR spectra were determined. An extensional study of substituent effects on the 1H NMR chemical shifts (δH(CH=N)) and 13C NMR chemical shifts (δC(CH=N)) of the CH=N bridging group from di-substituted to multi-substituted XBAYs was made based on a total of 182 samples of XBAYs, together with the NMR data of other 129 samples of di-substituted XBAYs quoted from literatures. The results show that the substituent specific cross-interaction effect parameter (Δ(∑σ)2) plays an important role in quantifying the δC(CH=N) values of XBAYs, but it is negligible for quantifying the δH(CH=N) values; the other substituent parameters also present different influences on the δC(CH=N) and δH(CH=N). On the whole, the contributions of X and Y to the δC(CH=N) of XBAYs are balanced, but the δH(CH=N) values of XBAYs mainly rely on the contributions of X.