Resonance Raman study of the structures of trans and cis isomers and the CC stretching frequencies of N,N′‐derivatives of indigo

The CC stretching frequencies of N,N ′‐derivatives of indigo reveal that although these compounds, with the exception of N,N ′‐oxalylindigo, are considerably twisted about the central CC bond owing to steric repulsion, the deviation from the coplanarity of the two halves of the molecule is not very different between the trans and cis isomers: the cis isomers are not always more twisted than the corresponding trans isomers. In contrast to the commonly accepted interpretation, the large hypsochromic shifts brought about by the trans → cis isomerization of N,N ′‐derivatives of indigo cannot be explained as being due to a departure from coplanarity in the cis configuration.