Raman and infrared spectra of the 7‐cisand di‐cisisomers of retinal

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Abstract
The Raman and infrared spectra of the 7‐cis, 9‐cis, 11‐cis, 9‐cis, 13‐cisand 11‐cis, 13‐cisisomers of retinal have been recorded for the first time. The spectra of the all‐trans, 9‐cis, 11‐cisand 13‐cisisomers were also recorded for comparison. The isomers were isolated from a photoisomerization mixture of the all‐transisomer and their configurations were identified by means of1H NMR or Raman spectroscopy. The Raman spectra of the isomers in CCl4solution were recorded using a flow method, the 457.9 nm line of an Ar+laser and a Raman spectrometer equipped with a multichannel detector; their infrared spectra were recorded by using an FT‐IR spectrophotometer. Comparison of the Raman spectra of the mono‐cisisomers revealed key bands which, discriminate between unmethylated‐cis(7‐cisand 11‐cis) and methylated‐cis(9‐cisand 13‐cis) configurations. The key bands were related to local vibrations around the particularcisdouble bond. Unmethylated‐cisconfigurations show (1) a unique band around 1280‐1270 cm−1due to a coupled mode ofA1type (with reference to the local symmetryC2νof thecis‐CCHCHC group) consisting of the two CH in‐plane bendings and the CC stretching, (2) a stronger band at 969 cm−1ascribable to anA2‐type coupled mode consisting of the two CH out‐of‐plane waggings and the CC torsion and (3) a unique band at 563 cm−1, tentatively related to anA‐type coupled mode consisting of the CCC and CCC deformations. Methylated‐cisconfigurations show a stronger Raman band around 880 cm−1due to CMe stretchings. Raman bands characteristic of eachcisconfiguration (7‐cis, 9‐cis, 11‐cisor 13‐cis) also are discussed.

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