University of Hyderabad

About: University of Hyderabad is a education organization based out in Hyderabad, India. It is known for research contribution in the topics: Thin film & Crystal structure. The organization has 6446 authors who have published 13005 publications receiving 237641 citations. The organization is also known as: Hyderabad Central University & HCU.

Supramolecular Synthons in Crystal Engineering—A New Organic Synthesis

Abstract: A crystal of an organic compound is the ultimate supermolecule, and its assembly, governed by chemical and geometrical factors, from individual molecules is the perfect example of solid-state molecular recognition. Implicit in the supramolecular description of a crystal structure is the fact that molecules in a crystal are held together by noncovalent interactions. The need for rational approaches towards solid-state structures of fundamental and practical importance has led to the emergence of crystal engineering, which seeks to understand intermolecular interactions and recognition phenomena in the context of crystal packing. The aim of crystal engineering is to establish reliable connections between molecular and supramolecular structure on the basis of intermolecular interactions. Ideally one would like to identify substructural units in a target supermolecule that can be assembled from logically chosen precursor molecules. Indeed, crystal engineering is a new organic synthesis, and the aim of this article is to show that rather than being only nominally relevant to organic chemistry, this subject is well within the mainstream, being surprisingly similar to traditional organic synthesis in concept. The details vary because one is dealing here with intermolecular interactions rather than with covalent bonds; so this article is divided into two parts. The first is concerned with strategy, highlighting the conceptual relationship between crystal engineering and organic synthesis and introduces the term supramolecular synthon. The second part emphasizes methodology, that is, the chemical and geometrical properties of specific intermolecular interactions.

Hydrogen bridges in crystal engineering: interactions without borders.

Abstract: A hydrogen bond, X-H...A, is an interaction wherein a hydrogen atom is attracted to two atoms, X and A, rather than just one and so acts like a bridge between them. This attraction always increases with increasing electronegativity of X and A, and in the classical view all hydrogen bonds are highly electrostatic and sometimes even partly covalent. Gradually, the concept of a hydrogen bond became more relaxed to include weaker interactions, provided some electrostatic character remains. In the limit, these weak hydrogen bonds have considerable dispersive-repulsive character and merge into van der Waals interactions. A great variety of hydrogen bonds are observed in the solid state and the aim of this article is to highlight some features common to all these bonds and further to suggest that the term hydrogen bridge is perhaps a better descriptor for them. Such a description recognizes an interaction without borders and one that admits of much variation in its relative covalent, electrostatic, and van der Waals content.

The C-h···o hydrogen bond: structural implications and supramolecular design.

Abstract: The C-H‚‚‚O hydrogen bond is so well-established in structural chemistry that it seems difficult now to believe that when Sutor proposed the existence of this type of hydrogen bond in the early 1960s,1,2 her suggestion was drowned in scepticism, if not outright hostility.3 It was only two decades later, with Taylor and Kennard’s paper, that the subject was properly revived.4 Shortly thereafter, an Account appeared from this laboratory describing the role of the C-H‚‚‚O interaction in crystal engineering.5 Subsequently, one felt confident enough to term these erstwhile “interactions” hydrogen bonds, in a second Account.6 A recent invitation to contribute another Account and the many recent efforts in this direction by my students and postdoctorals have led to the present paper. It is clearly no longer necessary to justify the relevance of C-H‚‚‚O hydrogen bonds, so widely invoked are they in small-molecule and biological crystallography. The presence of O-atoms in a large majority of organic molecules means that this hydrogen bond is widespread, even if not identified in many cases. However, other questions concerning these weak hydrogen bonds could be posed: (1) What is their upper distance limit? (2) Are very short, bent bonds significant? (3) Why do C-H‚‚‚O bonds sometimes disturb the strong O-H‚‚‚O and N-H‚‚‚O network? Alternatively, why do hydrogen bond donors and acceptors not always pair in descending order of strength? (4) How important is cooperativity for weak hydrogen bonds? (5) Are C-H‚‚‚O hydrogen bonds responsible for crystal packing, or are they the forced consequences of packing? (6) Are weak hydrogen bonds robust enough for supramolecular synthesis and crystal engineering? (7) Does the C-H‚‚‚O hydrogen bond have any biological significance? These difficult questions cannot be answered fully. This Account attempts to address some of them, but better answers can only follow from further work.

Crystal Engineering: A Holistic View

Abstract: Crystal engineering, the design of molecular solids, is the synthesis of functional solid-state structures from neutral or ionic building blocks, using intermolecular interactions in the design strategy. Hydrogen bonds, coordination bonds, and other less directed interactions define substructural patterns, referred to in the literature as supramolecular synthons and secondary building units. Crystal engineering has considerable overlap with supramolecular chemistry, X-ray crystallography, materials science, and solid-state chemistry and yet it is a distinct discipline in itself. The subject goes beyond the traditional divisions of organic, inorganic, and physical chemistry, and this makes for a very eclectic blend of ideas and techniques. The purpose of this Review is to highlight some current challenges in this rapidly evolving subject. Among the topics discussed are the nature of intermolecular interactions and their role in crystal design, the sometimes diverging perceptions of the geometrical and chemical models for a molecular crystal, the relationship of these models to polymorphism, knowledge-based computational prediction of crystal structures, and efforts at mapping the pathway of the crystallization reaction.