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

摘要

Quinacridones are industrially important red pigments characterized by NH...O intermolecular hydrogen bonds.Among these,unsubstituted gamma-quinacridone(gamma-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 gamma-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 gamma-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 gamma-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 gamma-QA,showing a higher stabilization of the former than the latter.