Half-life of radicals

Logarithm of Half-Life of Radicals as Function of Rate of Light Absorption J... 

If the fraction / is constant, 2 i// is a second-order rate constant, and the half-life of radical Ri is... 

Table III gives the half-life of radicals, for the same range of variables as are presented in Tables I and II. In an experiment with flash photolysis, the radical concentration may be about 1 mm. and the half-life is about 1 iMsec. In an experiment with a intense steady source close to the reaction vessel, the radicals may reach a concentration of lO Ycm. and the half-life is about 0.01 sec. In an experiment with weak sources or light sent through a typical monochromator, the radical concentration is typically 10 /cm. and the lifetime is 1 to 10 sec. In the latter case radicals will have time to diffuse to the walls, and thus two investigators may unwittingly be studying two totally different reactions even when they wanted to study the same photolysis, if one had a weak light source and wall reactions and the other had a strong light source with short-lived radicals.

Radicals can be detected with electron spin resonance (ESR) spectroscopy, which involves the detection of nnpaired electrons. The short half-life of radicals requires a complex sample preparation that consequently has an effect on the analysis. The method was proposed by the CEC Community Bureau of Reference for the detection of irradiated food (Raffi et al., 1992). 

In natural water, the half-hves fall between these extremes. For example, the half-life of Lake Zbrich water (pH 8, 1.5 meq/LHCO ) is 10 min (27). The decomposition in natural water also can be initiated by trace metal ions, eg, Fe , promoted by impurities such as organic matter, and inhibited by HO radical scavengers, eg, HCO3, COg , HPO (25,28). 

Entry 3 has only alkyl substituents and yet has a significant lifetime in the absence of oxygen. The tris(/-butyl)methyl radical has an even longer lifetime, with a half-life of about 20 min at 25°C. The steric hindrance provided by the /-butyl substituents greatly retards the rates of dimerization and disproportionation of these radicals. They remain highly reactive toward oxygen, however. The term persistent radicals is used to describe these species, because their extended lifetimes have more to do with kinetic factors than with inherent stability." Entry 5 is a sterically hindered perfluorinated radical and is even more long-lived than similar alkyl radicals. 

A novel cross-linked polystyrene-divinylbenzene copolymer has been produced from suspension polymerization with toluene as a diluent, having an average particle size of 2 to 50 /rm, with an exclusive molecular weight for the polystyrene standard from about 500 to 20,000 in gel-permeation chromatography. A process for preparing the PS-DVB copolymer by suspension polymerization in the presence of at least one free-radical polymerization initiator, such as 2,2 -azo-bis (2,4-dimethylvaleronitrile) with a half-life of about 2 to 60 min at 70°C, has been disclosed (78). 

These small-molecule thiols serve to transfer NO from erythrocytes to endothelial receptors, where it acts to relax vascular tension. NO itself is a reactive free-radical compound whose biological half-life is very short (1-5 sec). S-nitrosoglutathione has a half-life of several hours. 

Fewer compounds with Xe-C bonds have been characterized. The first to be claimed was synthesized by the plasma reaction of XeF2 with CF3 radicals the volatile waxy white solid produced, Xe(CF3)2, decomposed at room temperature with a half-life of about 30 min. " ... 

The triazolinyl radical 116 is thermally unstable with a half-life of -20 min at 95 °C. The compound 117 is stable under similar conditions. The decomposition mechanism involves loss of a phenyl radical and formation of a stable aromatic triazene (Scheme 9.26).24 This provides a mechanism for self regulation of the stable radical concentration during polymerization and a supplemental source of initiating radicals. 

The investigation by Becker et al. (1977 b) also included work on the effect of pyrene added as electron donor. Pyrene has an absorption maximum at 335 nm (e = 55000 M-1cm-1, in petroleum). Much more hydro-de-diazoniation takes place in the presence of pyrene with irradiation at 365 nm, and even more on irradiation with light of wavelength <313 nm. Photoexcited pyrene has a half-life of 300 ns and is able to transfer an electron to the diazonium ion. This electron transfer is diffusion-controlled (k= (2-3) X 1010 m 1s 1, Becker et al., 1977a). The radical pairs formed (ArN2 S +) can be detected by 13C- and 15N-CIDNP experiments (Becker et al., 1983, and papers cited there). 

For thermal reactions a variable temperature probe is necessary since optimum polarized spectra are usually obtained in reactions having a half-life for radical formation in the range 1-5 minutes. Reactant concentrations are usually in the range normally used in n.m.r. spectroscopy, although the enhancement of intensity in the polarized spectrum means that CIDNP can be detected at much lower concentrations. Accumulation of spectra from rapid repetitive scans can sometimes be valuable in detecting weak signals. 

The reactions are radical chain processes (Scheme 3) and, therefore, the initial silyl radicals are generated by some initiation. The most popular thermal initiator is azobisisobutyronitrile (AIBN), with a half-life of 1 h at 81 °C. Other azocompounds are used from time to time depending on the reaction conditions. EtsB in the presence of very small amounts of oxygen is an excellent initiator for lower temperature reactions (down to —78°C). The procedures and examples for reductive removal of functional groups by (TMSlsSiH are numerous and have recently been summarized in the book Organosilanes in Radical Chemistry. ... 

The current-potential relationship indicates that the rate determining step for the Kolbe reaction in aqueous solution is most probably an irreversible 1 e-transfer to the carboxylate with simultaneous bond breaking leading to the alkyl radical and carbon dioxide [8]. However, also other rate determining steps have been proposed [10]. When the acyloxy radical is assumed as intermediate it would be very shortlived and decompose with a half life of t 10" to carbon dioxide and an alkyl radical [89]. From the thermochemical data it has been concluded that the rate of carbon dioxide elimination effects the product distribution. Olefin formation is assumed to be due to reaction of the carboxylate radical with the alkyl radical and the higher olefin ratio for propionate and butyrate is argued to be the result of the slower decarboxylation of these carboxylates [90]. 

Very much more persistent radicals, with a half-life of up to a year, and with the structures [(Me3Si)2CH]3Sn- and [(MejSiljCHljRSn-(R = Pr , Bu , Me, Et, Bu, or cyclopentadienyl) have been prepared by the photolysis of [(Me3Si)2CH]2Sn(II), or by photolyzing a mixture of the appropriate halide [ Me3Si)2CH]2RSnX with an "electron rich olefin (299, 300). 

Most CO and CO2 in the atmosphere contain the mass 12 isotope of carbon. However, due to the reaction of cosmic ray neutrons with nitrogen in the upper atmosphere, C is produced. Nuclear bomb explosions also produce C. The C is oxidized, first to CO and then to C02 by OH- radicals. As a result, all CO2 in the atmosphere contains some 0, currently a fraction of ca. 10 of all CO2. Since C is radioactive (j -emitter, 0.156 MeV, half-life of 5770 years), all atmospheric CO2 is slightly radioactive. Again, since atmospheric CO2 is the carbon source for photos5mthesis, aU biomass contains C and its level of radioactivity can be used to date the age of the biological material. 

A free radical (often simply called a radical) may be defined as a species that contains one or more unpaired electrons. Note that this definition includes certain stable inorganic molecules such as NO and NO2, as well as many individual atoms, such as Na and Cl. As with carbocations and carbanions, simple alkyl radicals are very reactive. Their lifetimes are extremely short in solution, but they can be kept for relatively long periods frozen within the crystal lattices of other molecules. Many spectral measurements have been made on radicals trapped in this manner. Even under these conditions, the methyl radical decomposes with a half-life of 10-15 min in a methanol lattice at 77 K. Since the lifetime of a radical depends not only on its inherent stabihty, but also on the conditions under which it is generated, the terms persistent and stable are usually used for the different senses. A stable radical is inherently stable a persistent radical has a relatively long lifetime under the conditions at which it is generated, though it may not be very stable. 

The a- and [3-isomers of endosulfan undergo photolysis in laboratory tests after irradiation in polar solvents and upon exposure to sunlight on plant leaves. The a-isomer also undergoes isomerization to the P-isomer, which is relatively more stable (Dureja and Mukerjee 1982). A photolytic half-life of about 7 days was reported for endosulfan by EPA (1982c). The primary photolysis product is endosulfan diol, which is subsequently photodegraded to endosulfan a-hydroxyether. Endosulfan sulfate is stable to direct photolysis at light wavelengths of >300 nm however, the compound reacts with hydroxy radicals, with an estimated atmospheric half-life of 1.23 hours (HSDB 1999). 

Equation 6 would hold for a family of free radical initiators of similiar structure (for example, the frarw-symmetric bisalkyl diazenes) reacting at the same rate (at a half-life of one hour, for example) at different temperatures T. Slope M would measure the sensitivity for that particular family of reactants to changes in the pi-delocalization energies of the radicals being formed (transition state effect) at the particular constant rate of decomposition. Slope N would measure the sensitivity of that family to changes in the steric environment around the central carbon atom (reactant state effect) at the same constant rate of decomposition. 

Attention has been given to the possibility that some of the above motor effects may arise from a metabolite of levodopa. The prime suspect is OMD which has a half-life of some 20 hours and reaches plasma concentrations three- to fourfold those of dopa. Suggestions that it may compete with dopa for entry across the blood-brain barrier or act as a partial agonist (effective antagonist) have not been substantiated experimentally although it does reduce DA release from rat striatal slices. Also if free radical production through deamination of DA is neurotoxic (see below) then this would be increased by levodopa. 

Acrylonitrile is readily volatile, and significant quantities escape into air during manufacture and use. Volatilization may also occur from hazardous waste sites. In air, acrylonitrile is degraded primarily by reaction with hydroxyl radicals, with an estimated half-life of 5 to 50 hours. Acrylonitrile has been detected in air in the vicinity of various industrial sources at concentrations from 0.1 to 325 ppb, but has not been detected in typical ambient air. 

The principal pathway leading to degradation of acrylonitrile in air is believed to be photooxidation, mainly by reaction with hydroxyl radicals (OH). The rate constant for acrylonitrile reaction with OH has been measured as 4.1 x 10" cm /molecule/second (Harris et al. 1981). This would correspond to an atmospheric half-life of about 5 to 50 hours. This is consistent with a value of 9 to 10 hours measured in a smog chamber (Suta 1979). 

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