4,4-Diphenyl-2,5-cyclohexadienone: Four Polymorphs and Nineteen Crystallographically Independent Molecular Conformations

Abstract

Polymorphism is the existence of the same chemical substance in at least two different crystalline arrangements of molecules. [1] The existence of polymorphism implies that free energy differences between different crystalline forms are small (1±2 kcal mol À1 ) and that kinetic factors are important during crystal nucleation and growth. [2] Molecular conformations, packing arrangements, hydrogen bond synthons, and lattice energies of the same molecule in different crystalline environments can be compared in polymorphic structures. Polymorphs represent special situations for the study of structure±property relationships with a minimum number of variables, since differences arise from crystal packing effects and not because of the presence of a different chemical species. A proper understanding of polymorphism is important in crystal engineering [3] because the unexpected occurrence of a new polymorph can well demolish the best of crystal design strategies. Getting the correct polymorph [4] is of the utmost importance in the synthesis of drugs, pharmaceuticals, explosives, dyes, pigments, flavors, and confectionery products.We describe here four X-ray crystal structures of 4,4-diphenyl-2,5-cyclohexadienone (1) that exhibit conformational polymorphism, [5] conformational isomorphism, [5] and concomitant polymorphism [6] COMMUNICATIONS 3848

Exceptionally Large Difference in Bond Length among Conformational Isomorphs of a Hexaphenylethane Derivative with a Dispiropyracene Skeleton

“…Even when crystallographically independent sites are occupied by a pair of tautomers in different ratios, the observed differences in bond length between these sites are much smaller than those in the first case. The third case is related to conformational isomorphism, which has rarely been reported 24 probably because different conformers usually crystallize as polymorphs . In general, conformational isomorphs differ considerably in torsion angles but not in bond lengths because modification of bond lengths requires much more energy than modification of torsion angles, which cannot be compensated by the crystal packing force.…”

Stereochemistry Rules: A Single Stereocenter Changes the Conformation of a Cyclic Tetrapeptide

“…The presence of two molecular conformations 2a and 2b in the asymmetric unit ( Z ′) of 2 can be rationalized since, according to Desiraju, Z ′ > 1 structures occur when the crystals are “frozen” in some high-energy kinetic forms . The growing number of molecular conformations for Z ′ > 1 could also be considered as “snapshots” of the crystallization reaction . Both conformers 2a and 2b exhibit similar torsional angles of the peptide backbone (Table S2, SI).…”

“…48 The growing number of molecular conformations for Z′ > 1 could also be considered as "snapshots" of the crystallization reaction. 49 Both conformers 2a and 2b exhibit similar torsional angles of the peptide backbone (Table S2, SI).…”

Polymorphism in some new bis-hydrazone compounds

“…Bernstein and co-workers have extensively studied conformational polymorphism in different benzylidine aniline compounds (Bernstein, 1987;Matthews et al, 1991). Nangia and co-workers have described the conformational polymorphism of 4,4 0 -diphenylcyclohexa-2,5-dien- one (Kumar et al, 2002;Nangia et al, 2006) and bis(p-tolyl) ketone p-tosylhydrazone (Nangia et al, 2007). Conformational flexibility in a molecule arises due to torsional degrees of freedom which is a possible reason for the occurrence of conformational polymorphism.…”