Journal of Chemical Crystallography, Vol. 33, No. 4, April 2003 ( C  2003) Crystal structures of two thienyl analogs of benzil – 1,2-dithien-2-ylethanedione (2,2 ′ -thenil) and 1,2-dithien-3-ylethanedione (3,3 ′ -thenil) Guy Crundwell, (1)∗ James Sullivan, (1) Ryan Pelto, (1) and Katherine Kantardjieff (2) Received April 16, 2002 2,2 ′ -Thenil crystallizes in P 2 1 /c with a = 7.2501(12) ˚ A, b = 4.7846(8) ˚ A, c = 13.9867(23) ˚ A, β = 96.897(3) ◦ , V = 481.67(14) ˚ A 3 , and Z = 2. The molecule resides on an inversion center and is planar. 3,3 ′ -Thenil also crystallizes in P 2 1 /c with a = 3.9904(8) ˚ A, b = 21.310(4) ˚ A, c = 11.618(2) ˚ A, β = 101.83(3) ◦ , V = 966.9(3) ˚ A 3 , and Z = 4. Re- finement of 3,3 ′ -thenil data indicated that 10.3(2)% of both thienyl rings are flip-disordered in this nonplanar molecule. A brief discussion of disorder in molecules containing terminal, unsubstituted 2- and 3-thienyl rings is presented. KEY WORDS: Thenil; benzil; molecular structure; disorder. Introduction At room temperature, pure benzil crystallizes in P 3 1 2 (or P 3 2 2) and appears uniaxial when viewed normal to (001) faces. 1 However in the late 1800s, the French minerologist Wyrouboff reported that hexagonal crystals of benzil, when viewed down the c-axis, were biaxial and divided into six distinct and symmetrically related sectors upon inspection between crossed polars in a po- larizing microscope. 2 During the turn of the 19th century, many other optically anomalous organic crystals were reported by optical mineralogists. 3 Nearly a century later, some of these optically anomalous, sectored, organic crystals were dupli- cated by adding isomorphous impurities to puri- (1) Department of Chemistry, Central Connecticut State University, New Britain, Connecticut 06053. (2) W. M. Keck Center for Molecular Structure, Department of Chem- istry and Biochemistry, California State University, Fullerton, California. ∗ To whom correspondence should be addressed. E-mail: crundwellg@mail.ccsu.edu fied hosts; however, the cause of the anomalous birefringence that caused the stark sectoring in banzil has never been determined. 3 Researchers have used minor amounts of benzoin and deoxy- benzoin as impurities during benzil crystallization but were unable to recreate Wyrouboff’s curious observation. 3 Since thiophene has a similar molecular volume as benzene, guest molecules contain- ing thienyl rings could be used to examine host-guest interactions during crystal growth since guests can alter the morphology of host crystals and/or incorporate into the bulk of the crystal. 4 When forming mixed crystals, thienyl-containing guest have incorporated into the bulk by keeping non-aromatic based lone pairs on the thienyl-ring sulfurs pointed away from the face of the growing crystal presumably to avoid unfavorable electrostatic interactions. 5 Research in our lab has centered on syntheses and crystal structure of thienyl-analogs of phenyl- containing molecules commonly encountered in 239 1074-1542/03/0400-0239/0 C  2003 Plenum Publishing Corporation