[18]-annulene, C8H18, structure, disorder and Huckel's 4n+2 rule

An unexpected disorder has been discovered during an X-ray reinvestigation of the crystal structure of the monocyclic hydrocarbon C18H18. 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)Angstrom and six outer bonds of 1.419(4)Angstrom. The 0.037 Angstrom 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 degrees 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) Angstrom, outer bonds 1.405(3) Angstrom, C-C-C ring reentrant angles 127.9(4)degrees, C-C-C ring internal angles 124.0(2)degrees. 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 0.20 Angstrom too large. Thus, the 4n+2 pi-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: C18H18, M(r) = 234.34, monoclinic, P2(1)/a, a = 14.984(5), b = 4.802(2), c = 10.2.60(3) Angstrom, beta = 111.52(1)degrees, V = 686.8 (4) Angstrom(3), Z = 2, D-x = 1.134(1) Mg m(-3), lambda(Mo K alpha) = 0.71069 Angstrom,mu = 0.069 mm-1, T = 111 K, final R = 0.0398 for .1256 significant reflections.