Interpreting Substituent Effects on the Crystal Packing of Long-Chain Diacyl Peroxides. The Crystal Structures of D1 (11 Bromoundecanoyl) Peroxide and Di (Undecanoyl) Peroxide

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Abstract
Although crystals of di(11-bromoundecanoyl) peroxide and di(undecanoyl) peroxide have different space groups (P43212 and C2221), the molecules pack in almost identical layers. They differ only in the nature of stacking across interfaces involving the terminal groups. Because the 90° twist about the O-O bond locks neighboring molecules together within the layer, each peroxide shows a single solid phase from 5K to the melting point. Analysis of the stacking pattern in terms of the six possible orientational relationships suggests special stability for an L-shaped motif of C-Br-Br-C. Other substituents create different stackings of the same layer structure to give three crystal classes and five space groups among 14 compounds. Unsymmetrical peroxides are useful both for forcing a variety of substituted chains (particularly odd-even homologues) to pack with identical layer structures, and for controlling the stacking pattern. Because structural differences are localized in the vicinity of the substituents, this series of “substitutional polytypes” will allow systematic investigation of substituent effects on the physical and chemical properties of solids.