The intramolecular distances dC—Cl for chlorine bound to sp2 carbon and to sp3 carbon, respectively, and the 35Cl NQR frequencies assigned to the corresponding Cl atoms correlate quite well according to the theory: v(35Cl) ∼ (dC—Cl)−3. The relation is v(35Cl) = (4 ± 6) MHz + (166 ± 31) · 106 · (dC—Cl)−3 · MHz · pm3 for {sp2‐C}‐Cl and v(35Cl) = (21 ± 5) MHz + (101 ± 25) · 106 · (dC—Cl)−3 · MHz · pm3 for {sp3‐C}‐Cl. — The crystal structures and chemical bonds of 1 (4‐methyl‐4‐trichloromethyl‐2,5‐cyclohexadiene‐1‐one), 2 (2‐chloro‐4,5‐dimethyl‐4‐trichloromethyl‐2,5‐cyclohexadiene‐1‐one), 3 (4,5‐dichloro‐1‐phenyl‐pyridazine‐6‐one), and 4 (2,2,4,5‐tetrachloro‐cyclo‐pentene‐1,3‐dione) were studied by single crystal X‐ray diffraction and 35Cl nuclear quadrupole resonance (NQR). 1 crystallizes with the space group D152h‐Pbca, Z = 8. The lattice constants are a = 1206.0(3) pm, b = 1341.3(3) pm, and c = 1178.8(3) pm. 2 crystallizes monoclinic, space group C52h‐P21/c, Z = 4, a = 696.3(3) pm, b = 1404.0(5) pm, c = 1151.4(5) pm, and β = 100.23(1)°. 3 crystallizes in the orthorhombic space group D4h‐P212121, Z = 4, a = 4303(1) pm, b = 586.8(2) pm, and c = 390.5(1) pm. 4 crystallizes cubic, space group T4h‐Fd3, with 48 molecules in the unit cell, a = 2181.5(5) pm. The 35Cl NQR spectra were observed for the two 2,5‐cyclohexadiene‐1‐ones, 1 and 2 in the temperature range 77 < T/K < melting point. For 3, the 35Cl NQR spectra was determined at T = 77 K and T = 295 K. No phase transition has been found for the three compounds in the temperature range in which the 35Cl NQR spectroscopy was performed.

The 35Cl NQR spectra of the photochromic compounds 2,3,4,4‐tetrachloro‐1‐oxo‐1,4‐dihydronaphthaline. β‐TKN, and 2,4,4‐trichloro‐1,4‐dihydronaphth[1,8‐cd]isothiazole‐1,1‐dioxide, TCNS, were studied from 77 K to the decomposition temperature. The crystal structure of TCNS was determined, space group C‐P21/m, Z = 2, a = 818.3(2) pm, b = 828.0(2) pm, c = 905.4(2) pm, β = 107.552(2)°, σcalc = 1.752 Mg · m−3. No phase transition is observed for the two title compounds in the temperature range of the 35Cl NQR spectroscopy. The 35Cl NQR spectra are discussed in connection with the crystal structures. — For a few other compounds related to the photochromic ones, 2,2,3,4‐tetrachloro‐1‐oxo‐1,2‐dihydronaphthaline (α‐TKN), 2,3,4,4,5,6‐hexachloro‐cyclohexadiene‐(2,5)‐one‐(1) (HCP), 2,2,3,4,4‐pentachloro‐1‐oxo‐1,2,3,4‐tetrahydronaphthaline, and 2,3,4‐trichloro‐1‐naphthol, results of NQR spectroscopy are given, too.

The 35 C1 NQR spectra of the two phases (phases I and II) of 2,3-dichloro-l,4-naphthoquinone were studied from 77 to 394 K. The crystal structure determination of the triclinic phase (phase I) was refined by single crystal X-ray diffraction. Phase I crystallizes in the space group C/-P1. The unit cell (Z = 4) has the dimensions a = 1614.3(4) pm, 6 = 828.1 (3)

The 35Cl NQR spectra of 2,4,4,6-tetrachloro-3,5-dimethyl-cyclohexadiene-2,5-one-l and 2,4,4,6- tetrachloro-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 com­pound crystallizes in the monoclinic space group C22h -P21/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, β = 96.10(1)°. The sec­ond compound crystallizes orthorhombic, space group D182h - 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 deter­mined 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 chloro- cyclohexadiene-2,5-ones-1 and chloro-cyclohexadiene-2,5-diones-1.4. The intramolecular distances d(C-Cl) of the sp2 carbon atoms and the 35Cl NQR frequencies assigned to the corresponding Cl atoms correlate quite well according to the theory: ν(35Cl) ~ (d(c-cl))-3 The relation is ν(35Cl)/MHz = 2 + 174 · 106 -(d/pm)-3.

2,3,4,4-Tetrachloro-1-oxo-1,4-dihydronaphthaline, ß-TKN, crystallizes in the orthorhombic space group D1 6 2 h - Pnma with Z = 4, and the 35Cl NQR spectrum consists of three lines with an intensity ratio of 2:1:1. The crystal structure of 2,2,3,4-tetrachloro-1-oxo-1,2-dihydronaphthaline, α-TKN, is described in literature as belonging to the polar space group C22 - P 21 with Z = 2. In contradiction to the reported space group and Z, we observed a three line 35Cl NQR spectrum with an intensity ratio of 2:1:1. Therefore, the crystal structure of α-TKN was redetermined at 7 = 296 K: space group C22h-P21/m, Z = 2, a = 888.4 (2) pm, b = 692.3 (2) pm, c = 869.6 (2) pm, β = 91.483 (5)°. The nuclear quadrupole coupling tensors have been investigated using a 4 π Zeeman NQR goniometer and FT NQR for tracing the zero splitting cones. According to the crystal structure of β-TKN, two symmetry related zero splitting cones have been found for Cl(2), two for Cl(3), and four for Cl(4) ( T = 297 K). From the single crystal Zeeman split NQR measurements of α-TKN one zero splitting cone has been found for Cl(3), one for Cl(4), and two for CI(2) (T=290 K). The asymmetry parameters η and the direction cosines of the principal axes Φxx, and Φyy, and Φzz of the nuclear quadrupole coupling tensors have been determined. The results are: Cl(j), v (35Cl(j)), e2 Q q h -1 (35Cl(j)), η(35Cl(j)) for β-TKN (at T = 297 K): Cl(2), 37.442 MHz, 74.469 MHz, 0.184; Cl(3), 36.854 MHz, 73.083 MHz, 0.160; Cl(4) , Cl(4'), 38.685 MHz, 77.307 MHz, 0.070 and for α-TKN (at T = 290 K): Cl(2), CI(2'), 38.297 MHz, 76.501 MHz, 0.086; Cl(3), 36.943 MHz, 73.700 MHz, 0.123; Cl(4), 36.311 MHz, 72.314 MHz, 0.160. By comparing the crystal structures and the 35Cl NQR results, the C-Cl bonds in the title compounds are discussed.