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Self-Organization of 2,5-Di-n-Alkoxy-1,4-benzoquinones in the Solid State:  Molecular Recognition Involving Intermolecular Dipole−Dipole, Weak C−H···OC Hydrogen Bond and van der Waals Interactions

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journal contribution
posted on 14.05.1996 by Erik M. D. Keegstra, Valentijn van der Mieden, Jan W. Zwikker, Leonardus W. Jenneskens, Arie Schouten, Huub Kooijman, Nora Veldman, Anthony L. Spek
In the solid state 2,5-di-n-alkoxy-1,4-benzoquinones 1(n) organize into a two-dimensional layered structure with columnarly stacked 1,4-benzoquinone moieties along the a and b axis leading to improved intermolecular π-overlap. An interplay between intermolecular merocyanine-type dipole−dipole interactions, directional weak C−H···O hydrogen bond interactions and nondirectional van der Waals interactions is responsible for this packing motif. DSC of 1(n) shows that for n = 3−19 pronounced solid−solid phase transitions are discernible. Concomitant with alkoxy chain length odd−even effects are observed. Linear relationships are found between enthalpy and entropy changes, respectively, at distinct phase transitions for 1(n) with n ≥ 9 (n = odd) and n ≥ 10 (n = even) indicative for identical packing motifs in both series, respectively. Picnometry reveals that the densities of 1(n) with n = odd are higher than those for n = even. As shown by single-crystal X-ray structure analyses (1(3), 1(9), and 1(10)) and wide-angle X-ray diffraction (WAXD, 1(n), 9 ≤ n ≤ 19), this can be attributed to improved end-group packing of the alkoxy chains in the odd series.