Abstract
13C NMR spectra of a large number of crystalline retinal derivatives have been obtained by using cross-polarization and magic-angle sample spinning. Most derivatives yield spectra with narrow lines (width around 20 Hz) which can be assigned
via their l3C-'H dipolar coupling, their chemical shift tensors, comparison with solution spectra, or specific labeling. Measurement of the rotational sideband intensities in the spectra permit calculation of the chemical shielding tensors, and these data have allowed us to analyze variations in isotropic shifts of these compounds in more detail. We show that the tensors exhibit an odd/even effect which results from the steric crowding on one side of the polyene chain, that rr-electron perturbations affect primarily the in-plane elements of the tensor, and, conversely, that strong steric interactions due to cis-trans isomerization affect the out-of-plane element. Finally, we observe a downfield shift at the C-5 position on isomerization about the 6-7 bond, and thus deduce from the observed shifts that retinal derivatives are 25-30% 6-s-trans in solution.