Some derivatives of naphthaline show in the crystal line state photochromy and recently crystal structure studies and 35C1 nuclear quadrupole resonance, NQR, experiments on 2,3,4,4-tetrachloro-l-oxo-l,4-dihydro- naphthaline (ß-TKN) and 2,4,4-trichloro-l,4-dihydro- naphth[l,8-cd]isothiazole-l,l-dioxide (TCNS)

The 35C1 NQR spectra of 2,4,4,6-tetrachloro-3,5-dimethyl-cyclohexadiene-2,5-one-l and 2,4,4,6tetrachloro-cyclohexadiene-2,5-one-l were studied from 77 K up to the decomposition temperature, their crystal structure having been determined by single crystal X-ray diffraction. The first com­ pound crystallizes in the monoclinic space group C2h-P2!/m with two molecules in the unit cell. The lattice constants are a = 887.2 (3) pm, b = 704.8 (3) pm c = 824.0 (3) pm, y? = 96.10(1)°. The sec­ ond compound crystallizes orthorhombic, space group D^-Cmca, with 8 molecules in the unit cell which has the dimensions a = 711.2(2) pm, b = 1388.4(3) pm, c = 1729.8(4) pm. The structures deter­ mined belong to the stable phases of the title compounds as seen from the NQR spectra as a function of temperature which show no sign for a phase transition between the melting point and 77 K. In both title compounds the molecules are planar. Their symmetry is compared with other chlorocyclohexadiene-2,5-ones-l and chloro-cyclohexadiene-2,5-diones-l,4. The intramolecular distances d,c_ci) of the sp2 carbon atoms and the 35C1 NQR frequencies assigned to the corresponding CI atoms correlate quite well according to the theory: v(35Cl) ~(4c-ci))"3th e relation is v(35C1)/MHz = 2 +174 • 106-(d/pm)"3.