[18]-Annulene, C₁₈H₁₈, structure, disorder and Hückel's 4n + 2 rule

An unexpected disorder has been discovered during an X-ray reinvestigation of the crystal structure of the monocyclic hydrocarbon C 18 H 18 . The study at 80 K by Bregman, Hirshfeld, Rabinovich & Schmidt [Acta Cryst. (1965), 19, 227-234] resulted in an almost planar molecule with 12 inner C-C bonds of mean length 1.382(3) A and six outer bonds of 1.419(4) A. The 0.037A difference was and has remained puzzling. In new work at 111 K, partial disorder has been found. 86.3% of the molecules are in the orientation found by Bregman et al. and 13.7% are rotated by approximately 30° in the mean plane. Constrained refinement yields a molecule of approximate D 6h symmetry with mean dimensions in the major orientation : inner bonds 1.385 (2) A, outer bonds 1.405 (3) A, C-C-C ring reentrant angles 127.9 (4)°, C-C-C ring internal angles 124.0(2)°. The Bregman data is interpreted by a 6% disorder. The new dimensions agree well with the D 6h self-consistent field (SCF) calculations of Schulman & Disch [J. Mol. Struct. Theochem. (1991), 234, 213-225]. One can exclude a 50 :50 inversion disorder of D 3h molecules with alternating short and long bonds as their calculated ring diameters are up to 0.20A too large. Thus, the 4n + 2 π-electron molecule [18]-annulene is confirmed as having an aromatic structure in the crystal. The rotation disorder and small deviations from planarity are explained by the crystal packing. Crystal data : C 18 H 18 , M r = 234.34, monoclinic, P2 1 /a, a = 14.984 (5), b = 4.802 (2), c = 10.260 (3 ) A, β = 111.52 (1)°, V = 686.8 (4) A 3 , Z = 2, D x = 1.134 (1) Mg m -3 , λ(Mo Kα) = 0.71069 A, μ = 0.069 mm -1 , T = 111 K, final R = 0.0398 for 1256 significant reflections.