Crystals of 2,3,4,4-tetrachloro-1-keto-naphthalenedihydride (β-TKN) are orthorhombic, space group Pnma, a = 10.388 (±3), b = 6.955 (±3), c = 14.281 (± 4) Å. There are four molecules in the cell at special positions with symmetry m. The structure has been determined from 3252 reflections measured with an automatic three-circle diffractometer at −160°C. Anisotropic least-squares refinement gave Rw = 0.069. The geometry of the “benzene” ring of the molecule is similar to that of benzene itself, part of the remaining ring of the molecule shows a strong similarity with chloranil. The values observed for the C–Cl bonds are: C(sp3)–Cl = 1.800 (± 1), C(sp2)–Cl = 1.713 (±2) Å. Molecules at neighbouring mirror planes show a strong Cl…O interaction, Cl…O = 2.973 Å. Further electron-spin-resonance experiments are necessary to give an explanation of the photochromic behaviour of the crystals.
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