Crystal Engineering 3 (2000) 85–99 www.elsevier.com/locate/cryseng Polymorphism in a conformationally flexible substituted anthraquinone; a crystallographic, thermodynamic, and molecular modeling study Scott M. Reed, Timothy J.R. Weakley, James E. Hutchison * Department of Chemistry and Materials Science Institute, University of Oregon, Eugene, OR 97403-1253, USA Received 10 September 1999; accepted 10 May 2000 (Refereed) Abstract Single crystal X-ray structures of two polymorphs of 1-(1,6-dithiahexyl)anthracene-9,10- dione are reported. These structures reveal a conformational difference in the orientation of the hydrocarbon chain between the two polymorphs. This small change in the molecular con- formation dramatically influences the packing of the anthraquinone ring systems; in one case the anthraquinone rings exist as discrete pairs, and in the other they exist in infinite lamellar stacks. Differences in the packing results in different thermal behavior and spectroscopic properties of the two polymorphs. The yellow, triclinic structure is lower melting, whereas the orange, monoclinic structure is higher melting. Uses of this compound as a model for de novo computation of crystal structures and tuning of crystal packing through side chain vari- ation are discussed.  2000 Elsevier Science Ltd. All rights reserved. Keywords: Polymorphism; X-ray diffraction; Crystal structure; Thermodynamic properties 1. Introduction The existence of polymorphic forms of an organic compound provides unique opportunities for the investigation of structure-property relationships. The opport- unity to compare the physical properties of two polymorphic forms of a single com- * Corresponding author. E-mail address: hutch@oregon.uoregon.edu (J.E. Hutchison). 1463-0184/00/$ - see front matter  2000 Elsevier Science Ltd. All rights reserved. PII:S1463-0184(00)00031-9