Stable free radicals structure-reactivity

Marchantin H is a natural compound isolated from Marchantia diptera (Wu 1990). Marchantins are naturally phenolic structures isolated from different species of liverwort (Tori et al. 1985, Asakawa et al. 1987). Marchantin H could scavenge the stable free radical l,l-diphenyl-2-picrylhydrazyl and per-oxyl radical derived from 2,2 -azobis(2-amidino-propane) dihydrochloride in aqueous phase, but not the peroxyl radical derived from 2,2 -azobis (2,4-dimethylvaleronitrile) in hexane (Hsiao et al. 1996). It was reactive toward superoxide anion generated by the xanthine/xanthine oxidase system. Marchantin H inhibited copper-catalysed oxidation of human low-density lipoprotein, as measured by... 

In order to identify organic free - radicals present at quantifiable concentrations during the sonication of PCBs, we employed Electron Spin Resonance (ESR) with a spin trap, N-t-butyl-a-phenyl-nitrone (PBN). PBN reacts with the reactive free - radicals to form more stable spin-adducts, which are then detected by ESR. The ESR spectrum of a PBN spin adduct exhibits hyperfine coupling of the unpaired election with the 14N and the (3-H nuclei which leads to a triplet of doublets. The combination of the spin-adduct peak position and peak interval uniquely identifies the structure of a free-radical. 

Since only free radicals give an esr spectrum, the method can be used to detect the presence of radicals and to determine their concentration. Furthermore, information concerning the electron distribution (and hence the structure) of free radicals can be obtained from the splitting pattern of the esr spectrum (esr peaks are split by nearby protons).141 Fortunately (for the existence of most free radicals is very short), it is not necessary for a radical to be persistent for an esr spectrum to be obtained. Esr spectra have been observed for radicals with lifetimes considerably less than 1 sec. Failure to observe an esr spectrum does not prove that radicals are not involved, since the concentration may be too low for direct observation. In such cases the spin trapping technique can be used.142 In this technique a compound is added that is able to combine with very reactive radicals to produce more persistent radicals the new radicals can be observed by esr. The most important spin-trapping compounds are nitroso compounds, which react with radicals to give fairly stable nitroxide radicals 143 RN=0 + R —> RR N—O. 

In the case of the triphenylmethyl radical shown in Figure 4.86, it is possible to write many different resonance structures but in a small free radical such as the methyl radical there is only one possible structure. The reactivity of the radical decreases as the unpaired spin density at each site decreases, and the radical also becomes more stable because of the resonance energy. This resonance stabilization is zero for the phenyl radical, since the unpaired electron resides in an orbital which is orthogonal to the it system. By contrast, the methylphenyl radical has a resonance stabilization energy of some 10 kcalmol-1, and the larger methylnaphthyl radical is stabilized by about 15 kcalmol-1. These resonance stabilizations can have important consequences for the energy balance of photochemical reactions (see e.g. sections 4.4.2 and 4.4.4). 

Reactive free radicals also react with the nitrogen of nitroso groups, forming a nitroxide one atom closer to the trapped radical than is the case with nitrone spin traps. This results in ESR spectra containing more chemical structural information. While nitroso spin traps provide radical identification, the resultant adducts are often less stable than those derived from nitrone traps. In particular, nitroso traps are unreliable for oxygen-centered radicals even in vitro. 

However, there are indications that some structures are favorable for the material deposition more than others. According to the growth and the deposition mechanisms described in Chapter 5, the most important feature of organic molecules, which determines the ease of material deposition in LCVD, is the presence or absence of chemical structures that split to form diradicals by the energy transfer reaction with electrons or excited species in the luminous gas phase. The double bonds, triple bonds, and cyclic structures are the major chemical structures that create the reactive species that could proceed via cycle II of the growth and the deposition mechanisms (Figure 5.3). Without these structures, molecules depend on hydrogen abstraction or the detachment of simple stable species such as HF and HCl to create free radicals. 

The development of the physical organic chemistry of stable radicals stimulated the investigation of relationships between their structures and reactivity. As a result, there developed the widely held concept that organic free radicals which do not possess a system of conjugated multiple bonds, such as... 

For such short-lived radicals, the technique of spin trapping may be employed (Janzen, 1980). This technique involves the addition of the radical (R ) to an organic diamagnetic nitrone or nitroso compound to form a longer lived nitroxide free radical. The structure of the parent free radical may then be determined from the hyperfine coupling of the ESR spectrum of the resultant spin adduct. Nitrones are very reactive and their adducts are stable, even though they do not provide much structural information. On the other hand, nitroso adducts have unique ESR spectra but they are photolytically unstable. 

The second reason that vitamin C is used as an electron donor is that the reaction product is fairly stable and unreactive. When vitamin C gives up an electron, it becomes a free radical called the ascorbyl radical. By free-radical standards, the ascorbyl radical is not very reactive. Its structure is stabilized by electron delocalization — the resonance effect first described by Linus Pauling in the late 1920s. This means that vitamin C can block free-radical chain reactions by donating an electron, while the reaction product, the ascorbyl radical, does not perpetuate the chain reaction itself. 

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