Supramolecular thermodynamic stability

When rotaxanes and catenanes contain redox-active units, electrochemical techniques are a very powerful means of characterization. They provide a fingerprint of these systems giving fundamental information on (i) the spatial organization of the redox sites within the molecular and the supramolecular structure, (ii) the entity of the interactions between such sites, and (iii) the kinetic and thermodynamic stabilities of the reduced/oxidized and charge-separated species. 

Pluth, M.D., Bergman, R.G. and Raymond, K.N. (2007) Making amines strong bases Thermodynamic stabilization of protonated guests in a highly-charged supramolecular host. 

Selectivity is one of the main features in supramolecular chemistry and the search for receptors able to discriminate between one substrate from another has greatly motivated the synthetic developments in this area. A quantitative evaluation of the selective behaviour of a receptor for one species (ionic or neutral) over another can be obtained from the ratio of their thermodynamic stability constants in a given solvent and a given temperature. 

This chapter deals with formation of cyclic 2D and 3D structures in solution by self-assembly of two or more components using hydrogen bonds as the major interactions. Obviously, the formation of a defined supramolecular aggregate stabilized by non-covalent forces is a thermodynamically driven process which reflects a balance between enthalpy and entropy. Consequently, the product of a non-covalent macrocyclic synthesis must be evaluated and predicted in terms of thermodynamic minima in an equilibrium mixture. 

Another substantial factor directing the kinetic and thermodynamic stability of charged radicals is ion pairing. This phenomenon, although well estabhshed for many years, is often not directly distinct in experiments. To establish the importance of ion pairing, or, in other words, supramolecular interactions, a separate introductory chapter is dedicated to these aspects. 

An excellent, and far more thorough, treatment of binding constants can be found in Connors book of the same name [1] and an exhaustive treatment of supramo-lecular complexation thermodynamics has been undertaken by Schneider and Yatsimirsky [2], A highly relevant review comparing methods for determining supramolecular complex stabilities was also published in 1992 [3]. 

The rationale for the observed configuration (Scheme 3.29), is based on the X-ray structure of another a-carbamoyloxyorganolithium sparteine complex [185]. After deprotonation, the chelated supramolecular complex shown in the lower left is postulated. This structure contains an adamantane-like lithium-diamine chelate, and contains new stereocenters at the lithiated carbon and at lithium itself. Note that epimerization of the lithiated carbon would produce severe van der Waals repulsion between R and the lower piperidine ring, whereas epimerization at lithium produces a similarly unfavorable interaction between the same piperidine ring and the oxazolidine substituents. Thus, the carbamate is tailor-made for sparteine chelation of only one enantiomer of the a-carbamoyloxyorganolithium. These effects may provide thermodynamic stability to the illustrated isomer. To the extent these effects are felt in the transition state, they are also responsible for the stereoselectivity of the deprotonation. 

The field of supramolecular chemistry is concerned with a large number of systems ranging from simple host-guest complexes to more complicated solution assemblies, as well as two-dimensional (organized monolayers) and three-dimensional assemblies (crystalline solids). Nonco-valent interactions play an important role in the kinetic assembly and thermodynamic stabilization of all these systems and constitute their most distinctive feature. Electron-transfer reactions can obviously be affected by supramolecular structures, but the reverse is also true. It is possible to alter the structure and the thermodynamic stability of supramolecular assemblies using electrochemical (redox) conversions. In other words, electron-transfer reactions can be utilized to exert some degree of control on supramolecular aggregates. Provided in this article is an overview of the interplay between supramolecular structure and electron-transfer reactions. 

Thermodynamic stabilization reflects a drive to the lowest overall energy of the final phase. This tendency may cause flexible chemical species to change in order to accept the most favorable geometry in context of the resulting supramolecular environment. Thermodynamic stabilization falls into several types, depending on the kinds of species that are stabilized. The most common types are dealt with below. 

Figure 11 Hill plots are dose-response curves that describe the dependence of activity on monomer concentration. Hill analysis can differentiate between (a) unstable supramolecular active structures (n > 1 known stoichiometry, undetectable suprastructure) and (b) stable supramolecular or unimolecular active structures (n < 1 unknown stoichiometry, detectable suprastructure). Single channel lifetimes (t) differentiate between labile and inert active structures, whereas both open probabilities and Hill coefficients indicate thermodynamic stabilities.

In this chapter, we deal with Gd " " complexes applied as Ti agents in MRI. In the first part of the chapter, we discuss the different relaxation mechanisms and how supramolecular approaches can be used to optimize the efficacy of Gd " "-based contrast agents. Nontoxicity is of prime importance for in vivo nse of metal chelates therefore, we also shortly assess some aspects related to their thermodynamic stability and kinetic inertness. We include a short infioduction to CEST (chemical exchange saturation transfer) agents, a new class of lanthanide-based MRI probes, and discnss how supramolecular systems, particularly liposomes, can be beneficial to decrease the sensitivity limit of CEST detection. In the second part of the chapter. 

The examples discussed in this chapter illustrate that the dynamic covalent bond is now forming an integral part of the toolbox available to organic chemists. It also illustrates that the classical distinction between covalent and noncovalent bonds in terms of kinetic and thermodynamic stabilities is losing relevance. This, in turn, points to the important role supramolecular chemistry is playing in all areas of chemistry. 

Cellulose and its derivatives can form liquid crystalline solutions in a variety of organic solvents. Most of the lyotropic liquid crystalline phases derived from these compoxmds are cholesteric. Since the flow occurs in a shear field, the chiral nematic structure is transformed into a nematic phase. Nevertheless, shear phase orientation can be destroyed when the applied force is removed. This phenomenon is caused by the driving force that makes the liquid crystal form a supramolecular helical structure with thermodynamic stability [70]. The mesophase has a supramolecular helical structure, whose cellulose molecules are inclined at a small angle, which varies from one layer to another. 

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