A classification scheme relating the chirality of molecules to the type of crystal structures (chiral or achiral) they may form is presented. With respect to similar classifications proposed in the past, some corrections and extensions are introduced. In particular, (1) it is shown that chiral crystal structures from achiral molecules can occur in 28 types of space group having screw axes np
, with p ≠ n/2, not in any Sohncke type of space group; (2) it is shown that the restriction on Z′ > 1 for kryptoracemates is contradicted by examples with Z′ = 1; and (3) the case of scalemic enantioenriched solutions, absent from most classifications, is included. Chiral crystal structures from purely inorganic (non-molecular) compounds are addressed too.
In the title compound, the hydrated tetra(nitrate) salt of dapsone (4, 69.69 (19) . In the crystal, mixed cationanion-water molecule layers lying parallel to the (001) plane are formed through N-HÁ Á ÁO, O-HÁ Á ÁO and C-HÁ Á ÁO hydrogen-bonding interactions and these layers are further extended into an overall three-dimensional supramolecular network structure. Inter-ring -interactions are also present [minimum ring centroid separation = 3.693 (3) Å ].
Hybrid organic-inorganic compounds are receiving considerable attention in recent years due to the possibility of combining the different characteristics of the components to get unusual and enormous variety of interesting structural topologies and wide potential applications in the fields of catalysis, non-linear optics, sensors, magnetism and molecular recognition [1] . During our investigation, we synthesized three new hybrid organic-inorganic compounds of dapsone with antibacterial and second-order nonlinear optical properties [2] . The structural study and Hirshfeld surface analysis allowed us to establish the importance of hydrogen bond and intermolecular interaction in the crystal packing.and their role in the NLO properties.
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