Following our first experimental and computational study of the room temperature (RT) form of the tetrahydrated 3,6-dihydroxy-2,5-dimethoxy-p-benzoquinone dilithium salt (Li2DHDMQ·4H2O), we have researched the occurrence of hydrogen ordering in a new polymorph at lower temperature. The study of polymorphism for the Li2DHDMQ·4H2O phase employs both experimental (single crystal X-ray diffraction) and theoretical approaches. While clues for disorder over one bridging water molecule were observed at RT (β form), a fully ordered model within a supercell has been evidenced at 100 K (α form) and is discussed in conjunction with the features characterizing the first polymorphic form reported previously. Density functional theory (DFT) calculations augmented with an empirical dispersion correction (DFT-D) were applied for the prediction of the structural and chemical bonding properties of the α and β polymorphs of Li2DHDMQ·4H2O. The relative stability of the two polymorphic systems is evidenced. An insight into the interplay of hydrogen bonding, electrostatic and van der Waals (vdW) interactions in affecting the properties of the two polymorphs is gained. This study also shows how information from DFT-D calculations can be used to augment the information from the experimental crystal diffraction data and can so play an active role in crystal structure determination, especially by increasing the reliability and accuracy of H-positioning. These more accurate hydrogen coordinates allowed for a quantification of H-bonding strength through a topological analysis of the electron density (atoms-in-molecules theory).
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