The conformations of cis- and trans-isomers of benzylideneanilines: 1H nuclear magnetic resonance and optical spectroscopic studies

Abstract

The conformations of the cis- and trans-isomers of benzylideneanilines were studied by two spectroscopic approaches. Effects of trans–cis isomerization on diamagnetic shielding in the ¹H n.m.r. spectra suggest only a limited deviation of the aniline ring from the 1-α-α′-1′ plane in the trans-isomers in the absence of steric hindrance due to methyl substitution. Similar conclusions are suggested by the intensity-lowering effects of trans–cis isomerization and of methyl substitution on the first electronic absorption band. Such effects also indicate that the first electronic transition extends over the whole molecule as in stilbene and rule out the possibility that the aniline ring is perpendicular to the 1-α-α′-1′ plane in either isomer of benzylideneaniline.