Closed-shell compounds radicals

Termination reactions convert radicals to closed-shell compounds. Radical-radical coupling reactions are the reverse of homolytic cleavage reactions and are common, but radicals with (3-hydrogen atoms also react in disproportionation reactions as shown for 13. The selectivity of radical-radical terminations is low because the... 

Initiations Radicals from Closed-Shell Compounds. 140... 

The cage effect can be a source of great frustration in matrix isolation studies of monoradicals, because such species are usually formed by homolysis from closed-shell compounds, and hence any radical generated in situ is invariably accompanied by another radical that will be trapped in the same matrix cage. 

In early studies, flash vacuum pyrolysis, a method that has proven very valuable in preparative studies of closed-shell compounds,was regarded as the method of choice for the production of radicals for matrix isolation studies. " The disadvantage of this method, which is very well suited for preparative studies of closed-shell compounds, is that the reaction occurs on the walls of a hot tube whose surface may trap radicals (this problem may be alleviated by coating the inside of the tube with gold ). Also, unless a very low vacuum can be maintained in the pyrolysis mbe, collisions between radicals may lead to gas-phase dimerization. 

It should be remembered that it is a radical species that results from the addition of muonium to the close-shell compound that is studied in xSR. In studies of molecular, whole body dynamics where the muon appears to be an almost passive observer, it is the very light mass of the muon that makes possible for the observation of properties which are almost identical to those of the parent molecule. There are a number of such studies of organic systems [1]. jlSR studies of the dynamics of fullerenes Ceo and C70 are particularly interesting examples of such whole body motions where the change in the moment of inertia because of the added muonium is neglegible [27,28]. However the use of jlSR to study intramolecular dynamics presents at least two distinct scenarios. 

Awaga, K., Sugano, T., and Kinoshita, M., Ferromagnetic intermolecular interactions in a series of organic mixed-crystals of galvinoxyl radical and its precursory closed shell compound, J. Chem. Phys., 85, 2211, 1986. 

Radicals derived from hydrofluorocarbons (HFCs) as well as hydrofluo-roethers (HFE) are often destabilized with respect to the methyl radical [51, 57,68,70,79-82], The low stability of these radicals implies that the C-H bonds in the corresponding closed shell parent compounds are comparatively strong and thus rather unreactive towards attack of oxidizing reagents. This latter property is of outstanding importance for the use of these compounds in a variety of technical applications, in which thermally stable, non-oxidizable, non-flammable compounds are needed. However, with respect to the environmental fate of these compounds high C-H bond energies... 

When considering the stability of spin-delocalized radicals the use of isodesmic reaction Eq. 1 presents one further problem, which can be illustrated using the 1-methyl allyl radical 24. The description of this radical through resonance structures 24a and 24b indicates that 24 may formally be considered to either be a methyl-substituted allyl radical or a methylvinyl-substituted methyl radical. While this discussion is rather pointless for a delocalized, resonance-stabilized radical such as 24, there are indeed two options for the localized closed shell reference compound. When selecting 1-butene (25) as the closed shell parent, C - H abstraction at the C3 position leads to 24 with a radical stabilization energy of - 91.3 kj/mol, while C - H abstraction from the Cl position of trans-2-butene (26) generates the same radical with a RSE value of - 79.5 kj/mol (Scheme 6). The difference between these two values (12 kj/mol) reflects nothing else but the stability difference of the two parents 25 and 26. 

The data in Table 8.2 refer almost exclusively to closed-shell molecules. A second validation set for first-row compounds [42] contains 38 radicals and radical cations. The mean absolute errors for these species are higher than those in Table 8.2. They amount to 11.08, 9.73, 9.41, 6.70, and 4.79 kcal/mol for MNDO, AMI, PM3, OM1, and OM2, respectively. 

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