Building Upon Supramolecular Synthons : Some Aspects of Crystal Engineering
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Crystal engineering offers a rational way of analyzing crystal structures and designing new structures with properties. The supramolecular synthon concept was introduced in 1995 and has shown versatility and utility in the design of molecular solids. Chapter 1 gives a general introduction about the development of the concept of supramolecular synthons over the years which has seen a transition from synthesis to structures and dynamics. This thesis focuses on the later phase of the development of the concept of supramolecular synthons. Chapter 2 introduces the idea of structural landscape and describes a structural landscape of a conformationally flexible molecule, orcinol, and explores the synthon preferences of this particular molecule towards cocrystal formation. Chapter 3 explores a combinatorial matrix to show both global and local features of a structural landscape. Chapter 4 takes a component of this landscape namely 4,4'-bipyridine and 4-hydroxybenzoic acid and shows the occurrence of synthon polymorphism in cocrystals which originates from the interplay of geometrical and chemical factors. Chapter 5 introduces a four step method for the identification of multiple synthons by FTIR spectroscopy. Along with, it shows that the rarity of synthon polymorphism is not a case of overlooking of crystals in the process of selecting good looking crystals. Chapter 6 takes a series of dihalogenated phenols and indicates that the Br prefers type II. This chapter also explains elastic bending on the basis of halogen bonds. Chapter 7 attempts to explore the Cl/Br isostructurality in the light of type I and type II contacts and concludes that Cl/Br isostructurality arises from a geometrical model and therefore it is quite similar to Cl/Me isostructurality. Chapter 8 attempts to analyze the class of trichlorophenols and reveals structural modularity in this class of compounds. The modularity of 3,4,5-trichlorophenol is explored in crystal design in chapter 9 in terms of LSAM (Long Range Synthon Aufbau Module) A subsequent study in solution by NMR reveals the presence of LSAM in solution and establishes a hierarchy of the dissociation of its components. The concept of supramolecular synthon has come a long way from being a tool in a crystal engineer’s toolbox to a structural unit responsible for crystallization and therefore offer multiple possibilities both in terms of structures and dynamics. This thesis attempts to explore some of these possibilities based mainly on the concepts of structural landscape and halogen bonds which are blended with the concept of supramolecular synthons.
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