Chemical reactions steric aspects

When very high pressures (> 1 GPa) are applied to liquid phases, glasses, or molecular crystals, mobility is reduced and steric effects become more important both in equilibrium and in kinetic aspects. Equations (9) and (14) are still valid, but equilibria and kinetics of chemical reactions must take into account the energetic, structural, and dynamic properties of the environment as well. 

Which catalyst should be chosen for a given reaction will depend upon chemical, steric, and mechanistic factors. The application of Pearson s soft and hard acid-base (SHAB) principle has often proved a valuable qualitative guide as to suitable surface sites for a particular reactant. In fact, certain solids actually owe their catalytic power to attached Bronsted or Lewis acid and base groups as exemplified by weak acid ion exchange resins (Sect. 2.3), alumina (Sect. 3.2), and sometimes charcoals. Steric aspects can be con-... 

Studies on protein-free corrinoids and model complexes have shown that increasing the steric bulkiness around the coordinated Ca atom can cause a dramatic labilization of the Co—C bond. The protein-coenzyme adduct might contain the coenzyme in a resting state and the protein in a strained state the substrate would then switch the system into a strained coenzyme and a relaxed enzyme with little thermodynamic barrier. The strained form of the coenzyme is then in labile equilibrium with base-on cobalt(II) and the free radical. " This hypothesis, that conformational changes in cobalamin can switch chemical reactions on and off, is closely analogous with the known aspects of hemoglobin function. 

The chemistry of molecules consists of three major modules molecular architecture (structure) molecular dynamics (conformational analysis) and molecular transformation (chemical reactions). The molecular architecture consists of the basic principles of molecular structure and it deals with the atomic structure, orbitals, hybridization and bonding. Molecular dynamics deals with the molecular motion involving rotation around chemical bonds, steric interactions, torsional strain and properties associated with the conformational changes. Molecular transformation accounts for bond formation and bond breaking within the molecule or between molecules, which is generally called the chemical reaction, and consists of two major aspects, reaction mechanism and kinetics. The third module is one of the major areas of chemistry. This aims to understand the reaction mechanism and its manipulation to reduce the reaction barrier, improve stereoselectivity, increase product yield, or suppress undesirable side reactions. 

Sub-THz radiation - especially microwave radiation - is a powerful tool for initiating classical thermal chemical reactions (as noted in Section 1.2.2). The most obvious sub-THz radiation phenomenon relates to the significantly reduced reaction times, which can be interpreted in terms of the efficient and rapid heating of the reaction mixtures. However, many research groups are currently debating the existence of specific, so-called non-thermal modes of action regarding the initiation or modulation of chemical reactions. In particular, it has been asked whether the steric course and the chemoselectivity or stereoselectivity of reactions are altered by the influence of sub-THz radiation, as this would result in new products that would not be formed after classical heating. Various aspects... 

Asymmetric alkylation of carboxylic acid derivatives has been studied intensively for about 20 years. [1] Numerous auxiliaries, tailor-made structures with high steric demands for effective RelSi face differentiation, have been synthesized and their efficiency tested. [1, 2] In recent years besides the preparative aspects of enolates, physico-chemical investigations into their structure-reactivity relationships have gained interest. [3] Crystal structure analyses, osmometric measurements, and NMR studies in solution are helpful in the investigation of the factors that may control enolate reactions. [3-5]... 

One criterion for a reaction to be elementary is that as the reactants transform into the products they do so without forming intermediate species that are chemically identifiable. A second aspect of an elementary reaction is that the reverse reaction also must be possible on energy, symmetry and steric bases, using only the products of the elementary reaction. This reversible nature of elementary reactions is the essence of Tolman s principle of microscopic reversibility [37, p. 699]. 

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