Raman spectra of cis‐trans isomers of retinal homologues. Key bands of unmethylated cis configurations and of all‐trans parts of mono‐cis isomers

Raman spectra have been recorded for the cis‐trans isomers of retinal homologues. They include the all‐ trans , 9‐ cis , 11‐ cis , and 13‐ cis isomers of retinylideneacetaldehyde (C 22 aldehyde), which have a longer polyene chain than retinal ( C 20 aldeyhde), and the all‐ trans , 7‐ cis and 9‐cis isomers of β‐ionylidenecrotonaldehyde ( C 17 aldehyde) and β‐ionylideneacetaldehyde ( C 15 aldehyde), which have a shorter chain. The homologues were prepared by the stepwise elongation of the conjugated chain. The cis isomers were isolated by means of HPLC from a photoisomerization mixture of the all‐ trans isomer, and the configuration of each isomer was determined by 1 H NMR. The Raman spectra were recorded for flowing CCl 4 solutions under pre‐resonant conditions by using the 514.5 nm ( C 22 aldehyde) or 457.9 nm ( C 17 and C 15 aldehydes) line of an Ar + laser. Comparison of the spectra confirmed the key bands for the unmethylated cis configurations (7‐ and 11‐ cis ), distinguishing them from the methylated cis configurations (9‐ and 13‐ cis ), which were reported in a previous paper. It revealed also key bands for the all‐ trans parts of mono‐ cis isomers, i.e. the β‐ionone side and the carbonyl side.