Crystallization of a Racemate Affords a P21 Chiral Crystal Structure: Asymmetric Unit of Two Opposite Handed Molecules Simulates Achiral P21/n Packing via Pseudosymmetry

• Published in: Crystal growth & design 2011-04, Vol.11 (4), p.1262-1270
• Main Authors: Steinberg, Avital, Ergaz, Itzhak, Toscano, Rubén Alfredo, Glaser, Robert
• Format: Article
• Language: eng ; jpn
• Subjects: Condensed matter: structure, mechanical and thermal properties; Crystalline state (including molecular motions in solids); Equations of state, phase equilibria, and phase transitions; Exact sciences and technology; Physics; Solid-solid transitions; Specific phase transitions; Structure of solids and liquids; crystallography; Structure of specific crystalline solids; Theory of crystal structure, crystal symmetry; calculations and modeling
• ISSN: 1528-7483; 1528-7505
• DOI: 10.1021/cg1014994

A racemic mixture consisting of a secondary ammonium salt (±)-(1RS,3SR,4RS)-1-phenyl-cis-3,4-n-butano-5,6-dihydro-1H-2,5-benzoxazocine hydrochloride (1) crystallized as a “false conglomerate” of crystals in the monoclinic system, Sohncke space group P21 with two molecules of opposite handedness in the asymmetric unit and at 295(2) K: a = 10.224(2) Å, b = 13.969(2) Å, c = 12.724(2) Å, β = 98.996(2)°, V = 1794.9(5) Å3, Z = 4, and Z′ = 2. The cis-3,4-n-butano-5,6-dihydro-1H-2,5-benzoxazocine fused-ring skeletons are approximately enantiotopic and exhibit pseudoinversion and pseudo-n-glide relationships. These noncrystallographic symmetries enable space filling in the chiral crystal structure to resemble that of a higher order achiral P21/n apparent space group (Z = 4, Z′ = 1). The secondary ammonium salt molecules crystallize in patterns influenced by a complex blend of N−H···Cl, C−H···Cl, C−H···O, and C−H···Ar interactions that seem to be responsible for different conformational twists of the phenyl rings in the structure. Avnir's CSM method was adapted for quantification of crystallographic pseudosymmetry. RmS(G) measurements of distortion from ideal G symmetries were developed for pure translation, screw rotation, and glide reflection, as well as point group symmetries. Low rmS(i) and rmS(n-glide) values show high fidelity for the emulation of P21/n space filling in crystalline 1.