Phase transformation of propyl-[3-(pentamethyldisiloxanyl)propyl]dimethylammonium bromide

Abstract

Abstract The crystal and molecular structures of two polymorphic forms of propyl-[3-(pentamethyldisiloxanyl)-propyl]dimethylammonium bromide (C13H34N+OSi2 · Br−) have been determined by X-ray analysis using direct methods of phase determination. Both forms have the monoclinic space group P21/c with Z = 4. At room temperature form A with a = 19.051(5) Å, b = 8.390(2) Å, c = 13.542(5) Å, β = 104.00(3)°, V = 2100(1) Å3, Dx = 1.127 Mgm−3, μ(CuKα ) = 3.696 mm−1 was found. After cooling the crystal form B appeared, which was still stable after reheating. Form B has a = 18.42(2) Å, b = 8.390(3) Å,c = 13.57(1) Å, β = 95.98(6)°, V = 2085(3) Å3, Dx = 1.136 Mgm−3, μ(CuKα ) = 3.723 mm−1. In both polymorphs the molecules form bilayers which are held together mainly by N+ –Br− contacts. Form A is related to form B by shifting every bilayer parallel to the c-axis by c/2 relative to its neighboring bilayers. Because of the strong relation between both lattices the phase transformation can be regarded as topotactic reaction.