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Carbon-13 NMR spectra have been observed for 11- and 13-cis-retinals and their Schiff base linkage compounds with butylamine as a model compound of visual chromophore. The chemical shift changes on going from retinal to Schiff base. The changes in chemical shifts of polyene carbons were discussed in correlation with the changes of π-electron densities.
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Inoue et al. (1979) studied this question.
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