The crystal structure of the title compound has been refined from (a) three-dimensional, single-crystal X-ray diffractometer data and (b) three-dimensional photographic data. Crystals are orthorhombic, space group Pcab, with a= 8.263(2), b= 11.466(2), c= 15.238(2)Å, Z= 4. Starting with previously reported atomic parameters the structure was refined anisotropically (a) by full-matrix least-squares to R 0.035 (715 unique reflections), and (b) by block-diagonal least-squares to R 0.067 (1 148 unique reflections). Bond-lengths for each refinement are in good agreement with each other and with those predicted by Pauling's formula for single-bond-double-bond resonance. The molecule is centrosymmetric and planar.