Cohesion of γ-quinacridone and 2,9-dimethylquinacridone in the solid state

Quinacridones are industrially important red pigments characterized by NH⋯O intermolecular hydrogen bonds. Among these, unsubstituted γ-quinacridone (γ-QA) and 2,9-dimethylquinacridone (2,9-DMQA) are the major products used widely in painting and imaging industries. Our recent structure analysis revealed that the NH⋯O hydrogen bond in 2,9-DMQA is slightly weaker as compared with that of γ-QA, although the former is generally known to be more stable against heat and light irradiation than the latter. In order to clarify this point, the cohesion of γ-QA and 2,9-DMQA has been investigated on the basis of the energy partition analysis of semi-empirical molecular orbital calculations, using the X-ray x, y and z coordinate sets. The NH⋯O hydrogen bond is found to be the major intermolecular force in γ-QA and 2,9-DMQA, as expected. However, in 2,9-DMQA, an additional interaction is also recognized due to diagonal pairs along the stacking axis. Then, it follows that the sum of the hydrogen bond and the diagonal-pair interactions in 2,9-DMQA exceeds the total cohesive force of γ-QA, showing a higher stabilization of the former than the latter.