Optical Absorption Spectra of Organic Radicals

A large variety of organic radicals have been produced in irradiated aqueous solutions and monitored by their optical absorption spectra. Pulse radiolysis techniques allow the recording of such spectra and 

Optical Absorption Spectra of Radicals in Aqueous Solutions 

The recording of absorption spectra has also been applied to studies of acid-base equilibria, since radicals with dissociable functional groups usually show spectral shifts with pH which can be utilized for the determination of pAwalues. Most of the reported values of dissociation constants have, in fact, been determined in this manner. Several examples of spectral shifts can be found in Table 2 and a detailed discussion of acid-base equilibria of organic radicals is given in Section 6. 

Most substituted alkyl radicals have absorption maxima well below 250 nm so that the peaks have not been observed (see Table 2). The a-carboxy alkyl radicals show a distinct absorption with Amax in the region of 300-350 nm and e 1000 M 1 cm-1, in addition to another band with a maximum below 200 nm. Similar situations appear also when the carboxyl group is bound in an ester or amide. Hydroxyl and amino-groups bound at the radical site of a-carboxy-alkyl radicals diminish the absorbance in the 300-350 nm region and increase that at lower wavelengths up to e 5000 M 1 cm-1 at 250 nm. 

A considerable difference has been observed between the spectrum of cyclohexyl and that of the cyclopentyl radical, the former exhibiting a pronounced shoulder at 250 nm with e = 920 m cm-1. Cyclohexenyl and cyclopentenyl radicals show a much stronger absorption with definite maxima at 240 nm. These are allyl type radicals and like the allyl radical itself they show extinction coefficients of 7000-9000 M 1 cm-1. The optical spectrum of the allyl radical is greatly affected by unsaturated substituents which conjugate with the allylic 1 and 3 positions. These positions bear all the spin density and their interaction with carboxyl groups, for example, shifts Amax to 270 nm with extinction coefficients of 20,000-40,000 M-1 cm-1 (Neta and Schuler, 1975). A carboxyl group attached to the central carbon of allyl has only a minimal effect on the absorption. 

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Optical absorption spectra have not been utilized to measure these pA a values, although the radical cations of several phenols have been observed by pulse radiolysis and by laser flash photolysis in organic solvents. The spectra were found to be different than those of phenoxyl radicals, and in the presence of water they underwent very rapid deprotonation to form the corresponding phenoxyl radicals. 

Porphyrin radical cations were also produced by 7-irradiation in frozen organic media at 77 K. In hydrocarbon solvents, both the radical anion and the radical cation were formed from the porphyrin, but addition of a small amount of CCI4 eliminated the radical anion. Other experiments were carried out in chlorinated hydrocarbons. In this manner, the optical absorption spectra and the ESR spectra were recorded for the radical cations of chlorophyll, Pb TPP, Cu OEP, and V OTEP. The observed spectra led to the conclusion that in all these metalloporphyrins the species observed is the radical cation, i.e. the unpaired spin is predominantly on the porphyrin ring. Other metalloporphyrins, however, can be oxidized or reduced at the metal center. Such process are discussed below. 

An important property of EPR spectroscopy is the detailed information provided by the structure of the spectra so that there is higher degree of certainty of correctly identifying the radicals than is the case with optical spectroscopy. Thus, chemically similar radicals that would be expected to have similar UV absorption spectra have completely different EPR spectra. For example, in its reactions with organic molecules, OH can attack at a number of different sites, either to abstract a hydrogen atom from a saturated molecule (see Table 5) or to add to an aromatic ring. Each of the product radicals has a distinctive EPR spectrum, and the relative efficiency with which reaction occurs at each site on the target molecule can be determined from the intensities of the spectra of the various radical products. 

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