Competition between Halogen and Hydrogen Bonds in Triiodoimidazole Polymorphs

摘要

Three polymorphs of 2,4,5-triiodo-1H-imidazole (tIIm), all of exceptionally low symmetry (with three and four independent molecules), differ in the contributions of halogen bonds to their cohesion forces. The crystals of phase alpha, of monoclinic space group P2(1)/c and Z' = 3, were obtained after the first synthesis of the compound and several of its crystallizations. Above 1.9 GPa phase alpha isostructurally transformed to phase beta. The transition, marked by a huge strain visibly changing the crystal shape, could be reversibly repeated on increasing and releasing pressure. Repeated syntheses of tIIm, by using the same recipe as well as the same and varied conditions, resulted exclusively in a new gamma phase, of monoclinic space group P2(1)/c and Z' = 4, while phase alpha disappeared. Each of phases alpha, beta, gamma is built of Z' independent molecules and linked by Z' independent NH center dot center dot center dot N bonds into chains. In phases beta and gamma new I center dot center dot center dot I bonds are formed when one of the NH center dot center dot center dot N bonds is bent to about 130 degrees.