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p-Methoxycinnamic acid (I) is nematogenic but cinnamic acid (11), although as a hydrogen-bonded dimer it has the necessary molecular anisotropy, is not. The crystal structures of the two compounds have been determined at room temperature, and this difference can be directly attributed to differences in molecular packing.Crystals of (11) are monoclinic, space group P2Jc with a = 5. 644(1), b = 18.011 (6). c = 9.019(3) 8, @ = 121.47(2)", and Z = 4. The structure w a s solved by direct methods and refined by least squares to R 0.059 for 899 independent reflections whose intensities were measured by counter diffractometry. Crystals of (I) are triclinic, space group P i with a = 8.336(2), b = 12.026(4). c = 4.842(2) 8, M = 81.80(3), p = 99.28(2), y = 11 1.90(3)".and Z = 2. The structure w a s solved by a Fourier-transform method, and refined by least squares to R 0.048 for 1 675 independent reflections.The molecular packing in crystals of ( 11) is zig-zag type with the long axes of the hydrogen-bonded dimers inclined at 45". Crystals of (I) have all molecules parallel in a three-dimensionally interlocked arrangement characteristic of nematogenic crystals.THERMOTROPISM of the trans-isomers of the #-n-alk-8.336(2), b = 12.026(4), c = 4.842(2) A, cc = 81.80(3), p = * Observed and calculated structure factors for (I) and (11) are listed in Supplementary Publication No. SUP 21503 (20 pp., 1 microfiche.

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38) All least-squares refinements were based on the minization of Z W ,~ I F. 1 -I Fcl l p with the individual weights w, = I / U ( F . )~. Atomic scattering factors used for all nonhydrogen atoms are from Hanson, H. P.; Herman, F.; Lea, J. D.; Skillman, S. Acta Crystallogr. 1984, 17, 1040. Those for the hydrogen atoms are from Stewart, R . F.; Davidson, E. R.; Simpson, W. T. Abstract: The structures of the title compounds, Na4[M(C6H402)4]*21 HzO, M = Th, U, have been determined by single-crystal X-ray diffraction methods using counter data. These isostructural tetrakis(catecho1ato) complexes are structural archetypes for actinide-specific macrocyclic sequestering agents. The complexes have D2d molecular symmetry. The M -0 bond lengths are 2.417 (3) and 2.421 (3) 8, for Th and 2.362 (3) and 2.389 (4) 8, for U. The ring 0 -M -0 bond angles are 66.8 (1)'for Th and 67.7 (1)' for U. The difference in M -0 bond lengths for the uranium complex [0.027 ( 5 ) A] vs. a difference of 0.004 (4) A in the thorium complex is attributed to a ligand field effect of the 5fz electronic configuration of U(1V) vs_: 5P for Th(1V). Green crystals of the uranium complex, obtained from basic aqueous solution, conform to the space group I4 with a = 14.659 (3) and c = 9.984 (4) 8,. For 3660 independent data with FO2 > 3u(FO2) full-matrix least-squares refinement with anisotropic thermal parameters for all nonhydrogen atoms converged to unweighted and weighted R factors of 3.8 and 4.5%, respectively. The colorless thorium compound, also obtained from basic aqueous solution, has a = 14.709 (4) and c = 9.978 (3) 8,. For 5038 independent data refinement as above converged to unweighted and weighted R factors of 4.3 and 5.3%, respectively. Arthur F. Scott on the occasion of his 80th birthday. The structural studies reported here are supported by the NSF. The actinide sequestering project is supported by the Division of Nuclear Sciences, Office of Basic Energy Sciences, U S . Department of Energy. We are pleased to acknowledge the generous support by the U S . A.I.D. of K.A-D. We would like to thank Dr. Norman Edelstein for experimental assistance with the magnetometer and for several helpful discussions. Supplementary Material Atailable: Structure factor amplitudes and root mean square amplitudes o f vibration along principal axes (61 pages). Ordering information is given on any current masthead Page.

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The crystal and molecular structure of imidazo1ium tetrach1oro-dioxouranate{VI) has been determined by X-ray diffraction methods. The crystals, obtained by reacting imidazole and U0 2 C1 2-3H 2 0 in hydrochloric and maintained at pH = 2.5-3.0, are monoclinic,

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