Frontier Controlled Chemical Reactivity

As noted, the primary role for the electrons transfer in the molecular formation and in the development of the chemical reactions is, in broad sense, assured by the farthest electrons from the nuclei, or of those for whose action is the weakest felt. 

In other words, the valence electrons and their way of distribution have the dominant role in charge transfers. 

More specifically, the principal electronic levels are those named by HOMO (the highest-energetic-occupied molecular orbital) and LUMO (the lowest-eneigetic-unoccupied molecular orbital). As already shown, the combination between these two energy levels is representative for molecular systems behavior (i.e., for electrons interacting with a collection of nuclear charges) in their various combinations. 

However, the necessity for unification of concepts and of their submission to a Principle is natural, and this can be formulated in the context of the density functionals theory the forthcoming discussion follows the references (Parr Bartolotti, 1983 Parr et al., 1978 Parr G quez, 1993 Parr Yang, 1984 Yang et al., 1984)  

The CR4 principle affirms an essential aspect, beyond its sequential presentation, namely the prediction for the evolution of a chemical system (viewed as a collection of electrons and nuclei subject to the mutual interaction and with the environment too). 

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The model s dependence on ELUM0 and not on log Kow is reasonable because frontier orbital energies control chemical reactivity and ELUM0 energy can be considered as a measure of a compound s susceptibility to nucleophilic attack (Lynam et al., 1998). Previous relationships have been shown to be more influenced by electronic properties than by hydrophobicity (Damborsky and Schultz, 1997). 

The importance of both frontier orbital-controlled and electronic charge-controlled factors in determining chemical reactivity has been recognized (16). These concepts are the key to interpreting two types of reactivity expected for carbene complexes, i.e., reactions with nucleophilic... 

Klopman, G. 1968. Chemical reactivity and the concept of charge and frontier-controlled reactions. J. Am. Chem. Soc. 90 223-234... 

The existence of a charge density presupposes a concomitant set of MOs and Sect 2.3 describes how chemical reactivity can be based on the notion of Frontier Molecular Orbital (FMO) control i.e. the most important orbital interactions are between the HOMO on one species and the LUMO on the other or vice versa. FMO control, together with electrostatic charge control, provides a powerful qualitative basis for interpreting reactivity. The relative energies and compositions of MOs are vital and many computer programs now provide 3-dimensional representations of MOs to facilitate analysis. 

In order to understand the importance of frontier orbitals in chemical reactivity, Berkowitz [213] studied the frontier-controlled reactions within the purview of density functional theory. It is evident that the directional characteristics of frontier orbitals determine the extent of charge transfer, and soft-soft interactions are frontier-con-trolled. A somewhat similar analysis showed that charge transfer would be facilitated at a place where the difference in local softness of two partners is large [87], It may be noted that Fukui function is obtainable from local softness but the reverse is not true. On the other hand, local hardness suffers from the drawback of ambiguity [87], which allows one to even consider it to be equal to global hardness without disturbing their... 

Jacobs A, Hoy T, Humphrys J, Perera P (1978) Iron overload in Chang cell cultures biochemical and morphological studies. Br J Exp Pathol 59 489-498 Kattamis C (ed) (1989) Iron overload and chelation in thalassemia. Huber Toronto Klopman G (1968) Chemical reactivity and the concept of charge- and frontier-controlled reactions. J Am Chem Soc 90 223-234 Kontoghiorghes GJ, Bartlett AN, Hoffbrand AV, Goddard JG, Sheppard L, Barr J, Nortey P (1990) Long-term trial with the oral iron chelator l,2-dimethyl-3-hydroxypyrid-4-one (LI). I. Iron chelation and metabolic studies. Br J Haematol 76 295-300... 

Klopman, G. Chemical Reactivity and the Concept of Charge and Frontier-Controlled Reactions. J. Amer. Chem. Soc. 90, 223 (1968). 

This reactivity pattern is certainly unexpected. Why should low-valent complexes react as electrophiles and highly oxidized complexes be nucleophilic Numerous calculations on model compounds have provided possible explanations for the observed chemical behavior of both Fischer-type [3-8] and Schrock-type [9-17] carbene complexes. In simplified terms, a rationalization of the reactivity of carbene complexes could be as follows. The reactivity of non-heteroatom-stabilized carbene complexes is mainly frontier-orbital-controlled. The energies of the HOMO and LUMO of carbene complexes, which are critical for the reactivity of a given complex, are determined by the amount of orbital overlap and by the energy-difference between the empty carbene 2p orbital and a d orbital (of suitable symmetry) of the group L M. 

The static quantum chemical indexes of the reactivity may be used for description of kinetics of double exchange reactions which occur through the transition state with high charges separation [17. If a barrier height of the surface reaction is controlled by frontier orbitals interaction, then the following correlation may be fulfilled ... 

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