Frontier Molecular Orbital Theory radical reactions

The BDE theory does not explain all observed experimental results. Addition reactions are not adequately handled at all, mosdy owing to steric and electronic effects in the transition state. Thus it is important to consider both the reactivities of the radical and the intended coreactant or environment in any attempt to predict the course of a radical reaction (18). AppHcation of frontier molecular orbital theory may be more appropriate to explain certain reactions (19). 

Because of the early transition states in fast radical reactions, frontier molecular orbitals theory can be utilized for these reactions. 

These reactivity trends clearly show that polar effects are involved in these radical substitution reactions. The transition state is thought to include a charge transfer 9) from the radical (electron donor) to the pyridinium ion (electron acceptor) [13], Frontier Molecular Orbital Theory (FMO) [14] has been applied to explain the reactivity differences which have been observed upon varying the substituents at the pyridinium ion and upon altering the nucleophilicity of the attacking radical. Moreover, FMO can be used to explain the regioselectivities obtained in these homolytic aromatic substitutions. The LUMO of the substituted pyridinium cation... 

The BDE theory does not explain all observed experimental results. Addition reactions are not adequately handled at all, mostly owing to steric and electronic effects in the transition state. Thus it is important to consider both the reactivities of the radical and the intended coreactant or environment in any attempt to predict the course of a radical reaction (31). Application of frontier molecular orbital theory may be more appropriate to explain certain reactions (32,33). Radical reactivities have been studied by esr spectroscopy (34-36) and modeling based on general reactivity and radical polarity (37). Recent radical trapping studies have provided considerable insight into the course of free-radical reactions, particularly addition polymerizations, using radical traps such as 2,4-diphenyl-4-methyl-l-pentene (a-methylstyrene dimer, MSD) (38-44) and 1,1,3,3-tetramethyl-2,3-dihydro-liT-isoindol-2-yloxyl (45-49). 

Scheme 3-5). Ohya-Nishiguchi et al. (1980) noted that such a large localized spin density is very rare in a ir-electron system of purine s size and should have important application to its chemical reactivity. Reactions such as protonation should take place preferentially at position 6. This was deduced from the result of molecular orbital calculations (Nakajima Pullman 1959). According to Fukui s frontier electron theory (Fukui et al. 1952), such areaction should take place at the position where the frontier electron density is the largest. The calculations clearly indicate that the large electron density is at position 6. Scheme 3-5 describes the protonation of the purine anion radical (Yao Musha 1974). Protonation indeed takes place at position 6. After that, the radical center appears at the cyclic nitrogen in the vicinal 1 position.

Theoretical studies are also done to interpret the synthesis reactions and mechanism of reactions. The regioselectivity of 1,3-dipolar cycloaddition reaction between substituted trimethylstannyl-ethynes and nitrile oxides yielding isoxazoles, was interpreted by the application of frontier electron theory <93CPB478>. By the combination of experimental and molecular orbital (ab initio) studies, a multistep mechanism is proposed for unimolecular radical chemistry of isoxazoles in the gas phase <920MS(27)317>. 

Fukui, Kenichi (1918-98) Japanese physical and theoretical chemist. Fukui is best known for his work on frontier orbital theory, a theory which describes the changes in molecular orbitals during a chemical reaction. He was particularly interested in applying frontier orbital theory to the reactions of methyl radicals. He shared the 1981 Nobel Prize for chemistry with Roald HOFFMANN for his work on frontier orbital theory. He also studied the reaction between nitrogen molecules and transition metal complexes. 

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