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The crystal structures of 1,2,4,5‐Tetrachloro‐3,6‐dinitrobenzene and of 1,3,5‐Trichloro‐2,4‐dinitrobenzene were determined and the temperature dependence of the 35Cl NQR frequencies was investigated. 1,2,4,5‐Cl4‐3,6‐(NO2)2C6 crystallizes monoclinic, space group C32h‐C2/m with 2 molecules in the unit cell. The lattice constants are a = 990.4 pm, b = 923.4 pm, c = 573.5 pm, β = 92.721°. The plane of the nitro groups is perpendicular to the benzene ring. ‐ 1,3,5‐Cl3‐2,4‐(NO2)2C6H crystallizes monoclinic, space group C52h‐P21/c with 4 molecules in the unit cell. The lattice constants are a = 821.3 pm, b = 924.8 pm, c = 1300.0 pm, β = 92.39°. The nitro groups are tilted out of the ring plane (79.7° and 87.6°). — The 35Cl NQR spectra are in agreement with the crystal structures and in both cases no phase transition occurs in the temperature range 77 ≦ T/K ≦ melting point.

Nuclear quadrupole resonance is an effective spectroscopic method to investigate the charge density distribution in solids. The range of compounds which may be investigated is wide, reaching from a crystal lattice built up by an ordered arrangement of neutral molecules in the crystalline material to ionic crystals and to metallic solids.First the correlations between NQR quantities and bond parameters which become apparent from the molecular orbital picture will be discussed, e.g. Hammett a, x-parameter, ionic character i, s-character, ionicity, electronegativity in their connection to NQR. Then, the correlations of NQR frequencies, respectively nuclear quadrupole coupling constants, with bond lengths and bond angles are considered.Cross correlations, e.g. of NQR frequencies with bond distances, bond distances with vibrational frequencies, and vibrational frequencies with NQR frequencies are discussed as are pK A dependencies of NQR parameters. The connection of e

Chemical Bond / Molecular Structure / Spectroscopy, Nuclear Magnetic Resonance Resonance "CI and I4N nuclear quadrupole coupling tensors were determined at 295 K for 1,2,4,5-tetrachloro-3,6-dinitrobenzene and 1,3,5-trichloro-2,4-d1nitrobenzene. The Zeeman split single crystal "Cl NQR gives the quadrupole coupling constant e2qQh 1(3JC1) = 76.937(4) MHz and the asymmetry parameter q(3'CI) = 0.1207(5) for 1,2,4,5-tetrachloro-3,6-dinitrobenzene, and e2qQh-'(35C1"1) = 74.672(6) MHz, (20), e2qQh-1('5C1(5)) = 74.422(6) MHz, q(3'C1(51) = 0.1143(10) for 1,3,5-trichloro-2,4-dinitrobenzene. By high field NMR e2qQh-' (14N) and q(14N) were determined at 295 K . For 1,2,4,5-tetrachloro-3,6dinitrobenzene it is found e2qQh-'(14N) = 0.757(2) MHz, q(14N) = 0.025(3). QrZI('*N) deviates from the bond direction C-N by about 3.3", QXx is parallel to the benzene ring plane . For 1,3,5-trichloro-2,4-dinitrobenzene it is found: e2qQh-'('4N'2)) = 0.722(2) MHz, 4('4N(21) = 0.069(3), e2qQh-'('4N'41) = 0.738(2) MHz, q('4N(4') = 0.064(3). @zI (14N(21) and @zz(14N(4)) nearly coincide with the corresponding C -N bond directions. @xx('4N'4') is nearly parallel to the benzene ring plane, whereas @xx (14N(2)) is twisted about 18" against the ring plane.The relation between the twist angle Y and the asymmetry parameter of the ''N electric field gradient is discussed.

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