A Quantitative Examination of the Photoisomerisation of Retinal Iminium Salts by High-Field 1H NMR Spectroscopy

Article abstract of DOI:10.1021/ja00218a003

High-field ¹H NMR spectroscopy has been used to examine quantitatively the photoisomerizations of the N-tert-butyl, N-n-butyl, and N,N-dimethyl all-trans iminium salts of retinal, 1, 2, and 3, respectively.The NMR analytical method developed, which was shown to be more reliable than previously used hydrolysis/HPLC techniques, allowed for the composition of the starting mixtures to be assayed.Under the conditions chosen, CD2Cl2 as solvent and ClO4^- as counterion at 22 deg C, the various isomers of the retinylidene iminium salts were shown to be stable thermally.Irradiation of either 1, 2, or 3, using incident light of wavelengths 350, 436, or 468 nm, led in each case to the formation of only the 11-cis isomers as initial photoproducts.The quantum efficiencies, corrected for back-reaction by extrapolation to zero conversion, of the isomerizations of 1, 2, and 3 to the corresponding 11-cis isomers are 0.25, 0.17, and 0.18, respectively.Estimates of the quantum yields for the reverse isomerizations are given.