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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, 00:00 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.