13-cis-Beta,beta-carotene and 15-cis-beta,beta-carotene.

13-cis-Beta,beta-carotene, C(40)H(56), crystallizes with a complete molecule in the asymmetric unit, whereas 15-cis-beta,beta-carotene, also C(40)H(56), 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 molecule and the 6-s-cis beta 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) degrees. The molecules in each structure pack so that the arms of one occupy the cleft of the next, and there is significant pi-pi stacking of the almost-parallel polyene chains of the 15-cis isomer, which approach at distances of 3.319 (1)-3.591 (1) A.