13-cis-Β,Β-Carotene and 15-cis-Β,Β-carotene

13-cis-Β,Β-Carotene, C40H56, crystallizes with a complete mol-ecule in the asymmetric unit, whereas 15-cis-Β,Β-carotene, also C40H56, has twofold symmetry about an axis through the central bond of the polyene chain. The polyene methyl groups are arranged on one side of the polyene chains for each mol-ecule and the 6-s-cis Β end groups, with the cyclohexene rings in half-chair conformations, are twisted out of the planes of the polyene chains by angles ranging from 41.37 (17) to 52.2 (4)°. The mol-ecules in each structure pack so that the arms of one occupy the cleft of the next, and there is significant π-π stacking of the almost-parallel polyene chains of the 15-cis isomer, which approach at distances of 3.319 (1)-3.591 (1) Å. © 2008 International Union of Crystallography.