Structure, stability, reactivity

Diagrams like this ( reaction coordinate diagrams) provide essential connections between important chemical observables - structure, stability, reactivity and selectivity - and energy. These connections are explored in the following sections. 

In this section you have seen how heats of com bustion can be used to determine relative stabilities of isomeric alkanes In later sections we shall expand our scope to include the experimentally determined heats of certain other reactions such as bond dissociation energies (Section 4 16) and heats of hydrogenation (Section 6 2) to see how AH° values from various sources can aid our understanding of structure and reactivity... 

The opinion that stabilization of 1 by resonance was decisive, predominated for a long time and mastered the discussion of the relationship between structure and reactivity in free radical chemistry till quite recently5 Accordingly selectivity in free radical reactions was assumed to be mainly due to differences in the thermodynamic stability of the radicals taking part in a reaction or a potential competing reaction. 

With respect to the thermodynamic stability of metal clusters, there is a plethora of results which support the spherical Jellium model for the alkalis as well as for other metals, like copper. This appears to be the case for cluster reactivity, at least for etching reactions, where electronic structure dominates reactivity and minor anomalies are attributable to geometric influence. These cases, however, illustrate a situation where significant addition or diminution of valence electron density occurs via loss or gain of metal atoms. A small molecule, like carbon monoxide,... 

The intramolecular 1,2-H shifts of alkylchlorocarbenes are often very rapid making it difficult to relate structure with reactivity in terms of absolute rate constants. For example the ku values of Me2CHCCl, PhCHMeCCl, and EtCCl exceed 108 s 1 in hydrocarbon solvents at 25°C (Table 4).60 86 87 However, due to the stabilizing effect of the oxa spectator substituent, acetoxycarbenes react at much reduced rates relative to their chlorocarbene analogues,90,91 thus providing kinetically accessible results for a wide array of bystander-substituted alkylacetoxycarbenes.81 92... 

Molecular modeling seeks to answer questions about molecular properties— stabilities, reactivities, electronic properties—as they are related to molecular structure. The visualization and analysis of such structures, as well as their molecular properties and molecular interactions, are based on some theoretical means for predicting the structures and properties of molecules and complexes. If an algorithm can be developed to calculate a structure with a given stoichiometry and connectivity, one can then attempt to compute properties based on calculated molecular structure and vice versa. 

Aromaticity remains a concept of central importance in chemistry. It is very useful to rationalize important aspects of many chemical compounds such as the structure, stability, spectroscopy, magnetic properties, and last but not the least, their chemical reactivity. In this chapter, we have discussed just a few examples in which the presence of chemical structures (reactants, intermediates, and products) and TSs with aromatic or antiaromatic properties along the reaction coordinate have a profound effect on the reaction. It is clear that many more exciting insights in this area, especially from the newly developed aromatic inorganic clusters, can be expected in the near future from both experimental and theoretical investigations. 

An introduction to the non-covalent forces operating in stable ionic and molecular aggregates will be presented in Section 2. A brief description of the experimental methodologies employed in the production, detection, and characterization of clusters will be given in Section 3. The available experimental evidence on the structure of chiral clusters and their intrinsic stability, reactivity, and evolution dynamics will be presented and discussed in Sections 4 (molecular clusters) and 5 (ionic clusters). In the same sections, the experimental data will be interpreted in the light of the available theoretical evidence. Finally, some concluding remarks will be expressed in Section 6. 

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