STRUCTURE OF THE LOW-MELTING PHASE OF PETROSELINIC ACID

cis-6-Octadecenoic acid, C18H34O2, M(r) = 282.47, orthorhombic, Pbca, a = 7.311 (1), b = 5.565 (1), c = 88.01 (1) angstrom, V = 3581.1 (8) angstrom 3, Z = 8, D(x) = 1.05 g cm-3, lambda(Cu K-alpha) = 1.5418 angstrom, mu = 4.74 cm-1, F(000) = 1264, T = 263 K, R(F) = 0.054 for 1633 unique observed reflections with F > 3-sigma(F). Two bimolecular layers forming a double-layered polytype structure exist within a repeating unit along the c axis. The cis olefin group [C(5)-C(6) = C(7)-C(8)] has a 157, 0, -160-degrees conformation, deviating significantly from the standard skew, cis, skew' conformation. The hydrocarbon chains on either side of the olefin group adopt all-trans conformations and form an orthorhombic polyethylene-type subcell.