Third Generation Crystal Engineering : Supramolecular Synthons, IR Spectroscopy and Property Design

Crystal engineering is defined as “the understanding of intermolecular interactions in the context of crystal packing and in the utilisation of such understanding in the design of new solids with desired physical and chemical properties”. If crystals are the supramolecular equivalents of molecules, then crystal engineering is the supramolecular equivalent of organic synthesis. The subject considers both crystal structure analysis and design of new structures with targeted properties. The concept of “Supramolecular Synthons” was introduced by G. R. Desiraju in this context, for the rational design of structures. Supramolecular synthons are the smallest reducible structural units that contain geometrical and chemical information required for recognition between functional groups in molecular solids. Crystal engineering has grown very fast after the introduction of this idea in 1995 and engineered solids were found to be useful for application in many diverse fields, from structural chemistry to drug design. Because of the great significance of supramolecular synthons, their identification and analysis in terms of crystallographic, spectroscopic, and computational methods is essential. Single crystal X-ray diffraction (SCXRD) is a widely used technique for the identification of synthon structure. But the technique has its own limitations like requirement of good quality, suitably sized single crystals, longer times associated with the process which further restricts high throughput analysis. Practically, there is no other way for identification of synthons on a regular basis. In this situation a simple, accurate, and fast method will be of significance; not only for basic studies, but also to scan different solid state phases in pharmaceutical industries. Due to this reason, I have studied IR spectroscopy to find marker bands for different synthons in the first part of the thesis. In chapter 2, I have analyzed a variety of C–H···X based weak synthons. For identification of each synthon, two sets of compounds were taken. In one set the synthon exists and in the other set it does not. Comparison and verification of IR characteristics helps to establish marker bands. Such markers are used to get information on synthon patterns in compounds with unknown crystal structures. The next challenge is whether or not such an IR method can distinguish different geometries of a same interaction. To address this question, different geometries of NO2···I halogen bonded synthons are investigated in chapter 3. This synthon exists in three geometries P, Q and R based on angular and distance criteria. The identification process is divided into five steps. The first step identifies IR signatures from very similar compounds, but with different topologies. The second step verifies earlier features and establishes IR marker bands. In the next step a graded IR protocol is formulated for stepwise discrimination of unknown systems. Such a graded method is applied for clarification of synthon ambiguities and in the identification of synthons in…