Raman spectra ofcis‐transisomers of retinal homologues. Key bands of unmethylatedcisconfigurations and of all‐transparts of mono‐cisisomers

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Abstract
Raman spectra have been recorded for thecis‐transisomers of retinal homologues. They include the all‐trans, 9‐cis, 11‐cis, and 13‐cisisomers of retinylideneacetaldehyde (C22aldehyde), which have a longer polyene chain than retinal (C20aldeyhde), and the all‐trans, 7‐cisand9‐cisisomers of β‐ionylidenecrotonaldehyde (C17aldehyde) and β‐ionylideneacetaldehyde (C15aldehyde), which have a shorter chain. The homologues were prepared by the stepwise elongation of the conjugated chain. Thecisisomers were isolated by means of HPLC from a photoisomerization mixture of the all‐transisomer, and the configuration of each isomer was determined by1H NMR. The Raman spectra were recorded for flowing CCl4solutions under pre‐resonant conditions by using the 514.5 nm (C22aldehyde) or 457.9 nm (C17andC15aldehydes) line of an Ar+laser. Comparison of the spectra confirmed the key bands for the unmethylatedcisconfigurations (7‐ and 11‐cis), distinguishing them from the methylatedcisconfigurations (9‐ and 13‐cis), which were reported in a previous paper. It revealed also key bands for the all‐transparts of mono‐cisisomers, i.e. the β‐ionone side and the carbonyl side.