Supramolecular chemistry, description

Due to their unique electronic and chemical properties fullerenes have a tremendous potential as building blocks for molecular engineering, new molecular materials and supramolecular chemistry [54, 133], Many examples of fullerene derivatives (Section 14.1), which are promising candidates for nanotechnological or medical applications, have been synthesized already and even more exciting developments are expected. A detailed description of the potential of fullerene derivatives for technological applications would require an extra monograph. Since this book focuses on the chemical properties and the synthetic potential of fullerenes only a few concepts for fullerene based materials will be briefly presented. 

An attempt at a comprehensive description of the present status of the field, at least from the chemist s viewpoint, is being made in the multivolume Comprehensive Supramolecular Chemistry, (Eds. J. L. Atwood, J. E. D. Davies, D. D. MacNicol and F. Vogtle), Pergamon, in preparation. 

These descriptions, while helpful, are by their nature noncomprehensive and there are many exceptions if such definitions are taken too literally. The problem may be linked to the definition of organometallic chemistry as the chemistry of compounds with metal-to-carbon bonds . This immediately rules out Wilkinson s compound, RhCl(PPh3)3, for example, which is one of the most important industrial catalysts for organometallic transformations known in the field. Indeed, it is often the objectives and thought processes of the chemist undertaking the work, as much as the work itself, which determine its field. Work in modern supramolecular chemistry encompasses not just host-guest systems but also molecular devices and machines, molecular recognition, so called self-processes ... 

With the development of supramolecular chemistry, there has been a concomitant shift in the mind-set of chemists working in the area. This has involved a change in focus from single molecules, often constructed step by step via the formation of direct covalent linkages, towards molecular assemblies, with their usual (see exception above) non-covalent weak intermolecular contacts. This change in focus is nicely encapsulated in Lehn s description of supramolecular chemistry as the designed chemistry of the intermolecular bond . ... 

We must realize at this point that the calorimetric technique entirely builds upon the establishment of equilibrium conditions enabling the use and help from equilibrium thermodynamics. Many facets of supramolecular chemistry, however, do not comply with this prerequisite. Molecular recognition, for instance, in particular as a crucial property of all living matter, which exists because of nonequilibrium conditions, must be considered a process that relies on kinetic selechvity and thus per se is not open to an all-encompassing description of the phenomenon using this technique [12]. Similar arguments limit the utility of calorimetry in other vectorial processes like membrane transport, signaling, catalysis or locomotion. Never-... 

We have seen in the preceding sections that it is possible to explicitly model the molecular energetics and dynamics of a supramolecular assembly. Here, we shall briefly comment on how to calculate physical quantities that can be directly compared with experimentally measured observables, which is essential if theoretical descriptions and predictions are to be of any meaning. Of course, this endeavor is not special to supramolecular chemistry but is a general task in theoretical and computational chemistry. In supramolecular chemistry, however, new complications arise owing to the many particles involved. Therefore, it is necessary to selectively extract the relevant information needed to describe the features of the supramolecular assembly. 

The classical thermochemical approach has also already been applied to special template-assisted chemical reactions like the template-directed synthesis of oligonucleotides [260], for which a detailed system of kinetic elementary steps has been derived and solved. Other examples are the thermochemical model for the assessment of cooperativity in self-assembly processes proposed in Ref. [261, 262] and for the quantitative description of multicomponent self-assembly processes of polymetallic helicates [263, 264]. More thermochemical approaches applied within particular fields of supramolecular chemistry will be mentioned in the following. 

In this account we have attempted to provide a brief overview of the concepts of first-principles methods tailored for the calculation of structures, energetics, and properties of supramolecular assemblies. The presentation of the theory focussed on the most essential building blocks in order to provide a general frame to interrelate the various methods available. Thereafter, we discussed the relation of these methods to experiment and to well-known concepts for the description of typical interaction patterns. Also, new methods tailored for tackling problems specific to supramolecular chemistry have been discussed (like the calculation of local dipole moments in CPMD simulations, the Mode-Tracking protocol for the selective calculation of vibrational frequencies and intensities, or the SEN method for the calculation of hydrogen bond energies). 

Our aim in this work is to present a qualitative description of the properties of the most relevant intermolecular interactions from the point of view of the packing of molecular crystals. Therefore, we will not deal with the properties of metal-ligand interactions, at the root of much of supramolecular chemistry, as they do not play a leading role in the crystal packing of organometallic compounds. For the... 

Modern neutron diffraction is becoming an increasingly relevant tool for supramolecular chemistry. Allied to its natural advantages for the determination of hydrogen atom parameters, and thus for the fuller description of many nonbonded interactions, improvements in sources and instrumentation for neutron diffraction are now enabling these advantages to be realized in a wider range of systems. 

This book by lonel Haiduc and Frank Edelmann fills the need. It is particularly welcome for its timeliness and even more for its intrinsic qualities. Indeed, it provides a broad coverage, a thorough description, and incisive analysis of organometallic compounds and bonding features in terms of the concepts of, and relationships with, supramolecular chemistry. 

---