1H NMR spectra of the all- trans, 7- cis, 9- cis, 13- cis and 15- cis isomers of β-carotene: elongation of the double bond and shortening of the single bond toward the center of the conjugated chain as revealed by vicinal coupling constants

Abstract

400 MHz 1H-NMR spectra of the all- trans, 7- cis, 9- cis, 13- cis and 15- cis isomers of β-carotene have been recorded in benzene and chloroform solutions. A set of coupling constants for olefinic 1Hs has been obtained from the spectra of the two different kind of solutions, in which the chemical shifts and the overlap of the signals are different. Assignment of the signals newly made for the benzene solution is based on 1H 1H and 1H 13C chemical shift-correlated and 1H J-resolved spectra as well as the empirical rule of isomerization shifts. Comparison of the coupling constants for the cis-trans isomers reveals: (1) the vicinal coupling constant of the trans CC bond decreases, while that of the ( trans) CC bond increases, from the end toward the center of an extended conjugated chain; (2) the vicinal coupling constant of the cis CC bond in the center is smaller than that at the end. The changes in the coupling constants are ascribed to elongation of the double bond and shortening of the single bond, toward the center of the conjugated chain.