Isolated molecule theory reactivity

The electronic theory provides by these means a description of the influence of substituents upon the distribution of electrons in the ground state of an aromatic molecule as it changes the situation in benzene. It then assumes that an electrophile will react preferentially at positions which are relatively enriched with electrons, providing in this way an isolated molecule theory of reactivity. 

It is interesting to see how these facts are met by theory. We have seen (p. 16) that resonance theory describes pyrrole as a hybrid in which important contributing structures have concentrations of electronic charge at either the a- or / -positions. Inspection of these structures gives no help in deciding whether the a- or the j8-positions will be the more reactive (from the viewpoint of the Isolated Molecule Theory, p. 34) to electrophilic reagents, nor does it help to consider qualitatively the related Wheland transition states (67) and (68). The semi-quantitative molecular... 

In providing an isolated molecule description of reactivity, qualitative resonance theory is roughly equivalent to that given above, but is less flexible in neglecting the inductive effect and polarisability. It is most commonly used now as a qualitative transition state theory, taking the... 

The isolated molecule treatment of reactivity, which, in both the electronic theory and in m.o. theory, attempts to predict the site of electrophilic substitution from a consideration of the electron densities... 

For this reason, and because it applies equally not only to the effect of substituents but also to the large changes Sa, at an attacked atom— and hence to the reactivity indices used in the isolated molecule method, the theory of finite changes will be briefly outlined. 

Most of these developments may be applied most directly within the framework of the isolated molecule method, in which the reactivity indices are the charges and self-polarizabilities of the unperturbed ground state of a given molecule calculations based on the localization model (e.g. Nesbet, 1962) have made less progress, and will not be considered. It is therefore natural to enquire whether indices similar to and tt,, in Hiickel theory can still be defined, and calculated more precisely, in self-consistent field theory. The obvious questions are... 

Charge-transfer interactions between frontier molecular orbitals are clearly not the only factors which determine the relative stabilities of various transition states, in spite of the fact that frontier orbital theory has been remarkably successful in accounting for relative reactivities and regioselectivities in various reactions. For example, frontier molecular orbital theory is based on orbital shapes and energies present in the isolated molecules, and these are expected to change upon the approach of one molecule to another. 

These relationships link variations in the shape function to the reactivity descriptors commonly used in DFT and provide explicit methods for describing chemical reactivity in terms of the shape function yielding conceptual shape function theory . Thus, it is clearly seen that the shape function not only determines all the physical properties of an isolated molecule but, since reactivity descriptors can be explicitly constructed from the shape function, also its chemical properties. As the resulting equations are not always that simple to apply at first sight, we pass in next paragraph to some pragmatic procedures for extracting descriptors from the shape function. 

Another category is represented by some current reactivity theories which rely upon a large set of molecular indices, such as atomic populations, bond orders, free valency, autopolarizability, etc. These data represent an attempt to extract from the properties of the isolated molecule some useful information about its behavior as it interacts with other molecules. 

Carbon monoxide carbo-mers [5-7], i.e., monoxides of linear odd carbon chains longer than one C 0 n = 3, 5, 7, 9...), are highly reactive molecules suggested as potential constituents of interstellar and circumstellar gas clouds. Considerations based on MO theory and quantum chemical calculations indicate that, similar to pure odd carbon chains, all these heterocumulenes are singlet carbenes in the ground state [93]. Since its matrix isolation in 1971 [94] and its synthesis in gas phase in 1983 [95], the simplest member of this family, i.e., tricarbon monoxide C3O, has been extensively studied both experimentally and theoretically [96-102], and its interstellar presence fully confirmed [103]. In particular, on the basis of... 

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