Isoindoline packing

Table 14-3. Crystallographic data, packing coefficient PKC, calculated packing energy PE and specific packing energy SPE (PE/molecular mass) of isoindoline pigments..

P.R. 260 P.R. 260 crystallizes in the tridinic space group P 1 with 2 molecules in the asymmetric unit. The 2 molecular units (A and B) differ mainly in the conformation of the barbituric acid groups, which are skewed toward opposite sides of the molecular plane (Figure 14—9), while the isoindoline and the cyanomethylquinazo-lone unit are nearly planar. The combination of these two molecular conformations allows the maximum number of hydrogen bonds to form in the packing. 

O. 80 (0.74 is the PKC of identical spheres) l The packing coefficients of the investigated isoindoline pigments exceed this value (Table 14—3). Close packing is related to strong intermolecular interactions and must also be supported by the molecular shape. 

For comparison, a specific packing energy SPE = PE / mol. mass has been defined. The SPE values of the investigated isoindoline differs significantly. The specific packing energy of P.Y. 139 exceeds the corresponding value of P.R. 260 by 42 %, and thus marks P.Y. 139 as the intrinsically more stable solid. 

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