Free, electron molecular orbital theory radicals

The first indication of the existence of a captodative substituent effect by Dewar (1952) was based on 7t-molecular orbital theory. The combined action of the n-electrons of a donor and a captor substituent on the total Jt-electron energy of a free radical was derived by perturbation theory. Besides the formulation of this special stabilizing situation and the quotation of a literature example [5] (Goldschmidt, 1920, 1929) as experimental evidence, the elaboration of the phenomenon was not pursued further, neither theoretically nor experimentally. 

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). 

In this chapter, we review recent theoretical results of studies of radiation damage to DNA obtained in gas phase and in solution with the use of ab initio molecular orbital theory which has the advantage of being free of any empirical parameters. In the first part, we discuss results from works performed on species that are predominant in the direct effect, that is the natural DNA bases and their radical ions in various environments (i.e. base pairs, stacked systems, solvent), focusing on electron affinities and ionization potentials. We then review theoretical data obtained for species which result from OH and H attack in the indirect effect (sugar radicals, hydroxyl and hydrogen base adducts). In a third part, we discuss the fate of the hydroxyl base adducts, which upon H atom addition and subsequent dehydration lead to the regeneration of the natural DNA bases. Finally, we focus on the radioprotective roles of selected thiols and the possible mechanisms by which they act. Since an excellent and comprehensive review of the tools and methods currently used in molecular orbital theory has recently appeared, [15] it will not be further discussed here. 

Problem 8.28 (a) Apply the MO theory to the allyl system (cf. Problem 8.26). Indicate the relative energies of the molecular orbitals and state if they are bonding, nonbonding, or antibonding, (b) Insert the electrons for the carbocation C,H, the free radical C,H, and the carbanion CjH, and compare the relative energies of these three species. 

The early volumes of the Chemical Society s Quarterly Reviews also contain articles of value for the history of physical organic chemistry Maccoll (on colour and constitution) 401 Bell (on the use of the terms acid and base) 402 Coulson (on molecular orbitals) 403 and Hughes (on steric hindrance).404 The early volumes of Chemical Reviews similarly contain articles of value for the history of physical organic chemistry Gomberg (on free radicals) 405 Holleman (on factors influencing substitution in benzene) 406 Brpnsted (on acid-base catalysis) 407 Ingold (on electronic theories) 408... 

Molecules and ions with an unpaired electron are known as free radicals their existence can be explained by the molecular orbital (MO) theory of bonding (Chapter 14). Both nitrogen monoxide and nitrogen dioxide exist as resonance hybrids (see pages 134-135) of two Lewis structures. 

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