The Crystal Structure of 13-cis-Retinal. The Molecular Structures of its 6-s-cis and 6-s-trans Conformers

Article abstract of DOI:10.1107/S0567740881008388

The crystal and molecular structure of 13-cis-retinal, C20H28O, has been determined by single-crystal X-ray diffraction techniques using counter methods.The structure was refined by full-matrix least-squares procedures using 2762 unique and significant (at the 2? level) reflections to a final weighted R index of 0.062.Triclinic crystals form in the space group P1/ with unit-cell dimensions of a=12.494(6), b=18.279(8), c=7.992(5) Angstroem, α=100.26(4), β=90.26(5), γ=94.35(4) deg, with V=1790(2) Angstroem³, Z=4, D_c=1.055 Mg m^-3, M_r=284.4, F(000)=624.The crystal structure discloses the presence of two conformers.In the 6-s-trans conformer, the double bond in the cyclohexene ring lies nearly in the plane of the polyene chain of the molecule, while in the 6-s-cis conformer, the ring is rotated 110 deg from this orientation.Although the greater extent of the conjugated system in the 6-s-trans conformer indicates that it has more resonance stabilization than the 6-s-cis conformer, its intramolecular contacts and distortions show that it also has a higher steric energy.For the two conformers to coexist in the crystal, these two energy differences must nearly balance.