Structures for polymorphs of triphenylchloromethane. Triphenylacetic acid illustrates the isomorphous stable modification

Triphenylacetic acid is isomorphous with the published structure of triphenylchloromethane (I). It crystallizes in the trigonal system, space group P3, with a=b=14.105(3)Å c=13.121(2)Å, V=2261(1)ų, Z=6 at 175K. The structure was solved by direct methods and refined to R(F_o)=0.062. We determined the structures for two triclinic polymorphs of triphenylchloromethane, II and III. The three forms were obtained from pentane evaporated at room temperature. Phases I and III were isolated from particular solvent systems. Crystal data (II): triclinic system, space group P1, with a=13.028(l)Å, b=13.559(2)Å, c=13.969(3)Å, α=l 17.67(1)°, β=92.67(l)°, γ=91.43(l)°, V=2179.9ų, Z=6 at 228K. The structure was solved by direct methods and refined to R(F_o)=0.045. Crystal data (III): triclinic system, space group P1, with a=14.1562(4)Å, b=21.3190(7)Å, c=13.0654(5)Å, α=99.92(3)°, β=92.68(3)°, γ=106.15(2)°, V=3712(2)ų, Z=10 at 248K. The structure was solved by direct methods and refined to R(F_o)=0.064. A batch of crystals from pentane showed three peaks in the DSC thermogram at 99°C (III), 108°C (II), and 111°C (I). Phase III melts without being transformed to phase II or phase I. Phase II could not be isolated for calorimetric studies.