A New Aspect on the Origin of an Anomalous Substituent Effect on the Azomethine Proton Chemical Shifts in 4-Substituted N-Benzylideneanilines

Carbon-13 chemical shifts, azomethine proton chemical shifts, and azomethine carbon–hydrogen coupling constants have been determined for N-benzylideneaniline and its several derivatives bearing o-methyl and/or p-nitro groups on the aromatic ring on the nitrogen atom (ring A). The anomalous upfield shift of the azomethine proton caused by introduction of a 4-nitro group on the ring A can be attributed to the increase in the twist angle θN of the ring A from the molecular plane containing the C=N bond.