Radical cations decay

Hydroxyl radicals were generated radiolytically in NaO-saturated aqueous solutions of thiourea and tetramethylthiourea. Conductometric detection showed that HO and a dimeric radical cation were produced. The dimeric radical cation is formed by addition of a primary radical to a molecule of thiourea. In basic solution, the dimeric radical cation decays rapidly to a dimeric radical anion, which is formed via neutralization of the cation and subsequent deprotonation of the neutral dimeric radical (Scheme 16). This was not observed in tetramethylurea. These dimeric radical cations of thiourea and tetramethylurea are strong oxidants and readily oxidize the superoxide radical, phenolate ion, and azide ion. 

Analysis of the transient absorption spectra recorded at 100 ns allows for the determination of the prompt relative yields of the G /G(-H) radicals, I g (<100 ns). This yield is probably due, in part, to the oxidation of the guanines by the 2AP + radical cations [11]. Assuming that the 2AP radical cations decay only via the deprotonation of 2AP (rate constant ku) and hole transfer from 2AP + to guanine (fct), the prompt yield Og may be expressed as follows ... 

Considerable stabilization is achieved by two methoxy groups, and the radical cation resulting from reaction (49) is stable with respect to its reaction with water (k < 103 s4) (Behrens et al. 1980). Its radical properties are not altered by the stabilization of the cation, and hence these radical cations decay bimolecu-larly at diffusion-control rates. 

The polarization observed in the photoreaction of anthraquinone is also consistent with the concept of two consecutive intermediates [178]. Anthraquinone is a poorer oxidizing agent than benzoquinone so that the corresponding radical ion pair is less stable. Consequently, the aminium radical cation decays faster and does not contribute appreciably to the vinylamine polarization. 

The radical cation decayed in the time scale of hours the reaction was attributed to an intramolecular conversion of the radical cation into a benzyl type radical, which subsequently decayed to produce stable recombination products. 

They are characterized by an intense absorption band in the UV region ( 300 nm) and a broad, less-intense band in the visible region (from 500 to 1000 nm) whose position depends on the nature of the ring substituents H/550 nm, OCHj/OOO nm, CH3/67O nm, Br/660 nm, and CN/560 nm. At low radiation doses the radical cations decayed by first-order kinetics with the decay rate increasing with pH. The decay involves a nucleophilic attack of H2O or OH on the positively charged sulfur atom resulting in the formation of [ArSO(OH)CH3], as suggested earlier for dialkyl sulfoxide radical cations. ... 

In the absence of suitable redox partners sulfide and disulfide radicals cations decay mainly by disproportionation or deprotonation. Considering their positive charge they are also prone for nucleophilic attack. Examples for the latter are the reaction of R2S with OH , leading to sulfuranyl radicals R2S (0H), or with halide ions, yielding sulfur-halide coupled radicals. Both these product radical species will be dealt with in more detail in separate sections. 

As follows from Figure 7.3, the experimental rates (dots) are proportional to [NOj ]o at relatively low concentrations of NO (< 7 x 10" mol/1). As the concentration of NO is increased, the plot drastically deviates from linearity because of an increase in the probability of intermediate radical cation decay in side reactions of oxidation with nitrosonium cations through the following reaction ... 

An evaluation of thermodynamic and kinetic parameters for S S bonds in solution has been derived from kinetic analysis of the (R2SaSR2) radical cation decay as a function of sulfide concentration and deliberate acceleration of the R2S deprotonation by addition of a proton acceptor such as, for example, (HP04 ) [117]. Reactions (57)-(59) are to be considered ... 

The decay of zinc tetrakis(A -methylpyridinium-4-yl(porphyrin 7r-radical cations in the presence of poly(styrenesulfonate) and IrIV oxides or [Ir(OI I )r,]2 has been studied at high pH.46 The hexahydroxide species was found to be very reactive. 

Radical cations can also be produced in solution by photoinduced electron transfer (PET) in polar solvents. Although this method is widely used to study the processes involved in the formation and decay of ion pairs224, free radical cations appear only as transients in such experiments. 

Bobrowski and Das33 studied the transient absorption phenomena observed in pulse radiolysis of several retinyl polyenes at submillimolar concentrations in acetone, n -hexane and 1,2-dichloroethane under conditions favourable for radical cation formation. The polyene radical cations are unreactive toward oxygen and are characterized by intense absorption with maxima at 575-635 nm. The peak of the absorption band was found to be almost independent of the functional group (aldehyde, alcohol, Schiff base ester, carboxylic acid). In acetone, the cations decay predominantly by first-order kinetics with half life times of 4-11 ps. The bimolecular rate constant for quenching of the radical cations by water, triethylamine and bromide ion in acetone are in the ranges (0.8-2) x 105, (0.3-2) x 108 and (3 — 5) x 1010 M 1 s 1, respectively. 

Upon irradiation of an IPCT band in degassed condition (Xex> 365 nm), the colour of both LB films changed from pale yellow to blue. The UV/vis absorption spectrum after irradiation is shown in Figure 25, which is characteristic of 4,4 -bipyridinium radical cation monomer[108]. Coloured species photogenerated in mixed LB films of AV2+/AA or HV2+/AA systems decayed almost exponentially in the dark in vacuo with a lifetime of about 4 h at 20 °C [93,94]. The lifetime of 4,4 -bipyridinium radical cations in LB films was almost... 

Figure 26 The p-polarized absorbance of 4,4,-bipyridinium radical cations in LB films at 400 ran after correction of the decay and optical path length for photoexdted (a) HV2+/AA and (b)AV2+/AA systems. The solid lines are calculated dependences.

In order to measure the absorption spectra, the radical anions were generated electrochemically in the optical path of a spectrophotometer. The absorption spectrum of 3,5-dinitroanisole radical anion (Figure 11, curve c) is very similar to that of the 550-570 nm species produced photochemically. So we believe this species to be the radical anion formed by electron transfer from the nucleophile to the excited 3,5-dinitroanisole and decaying by interaction with its surroundings including the nucleophile radical cation. The behaviour described seems to be rather general for aromatic nitro-compounds since it is observed with a series of these compounds with various nucleophilic reagents. 

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