[16]Annulene: the crystal and molecular structure

Abstract

The crystal structure of [16]annulene has been determined by two independent groups. One determination was at 4° with equi-inclination Weissenberg film X-ray data, and was refined to an R of 0·12 on 1204 observed reflexions; the other determination was at 23° with counter data and has been refined to an R of 0·06 on 822 reflexions. The crystals are monoclinic, a= 8·779, b= 8·152, c= 18·643 Å, and β= 109° 30′, (room temperature data), Z= 4, space group P2₁/c. The molecule is non-planar, with almost complete bond alternation. The single bonds (1·454 Å) are alternately trans and gauche and the double bonds (1·333 Å)cis and trans. The average torsion angle at a gauche bond is 41° so that the molecule is relatively flat with S₄ non-crystallographic symmetry. These determinations confirm the lack of aromaticity of this [4n]annulene.