Phenyl radical electronic absorption spectrum

Case Studies Electronic Absorption Spectrum of Phenyl Radical... 

In the decay of the radical cation of 4-methyldiphenylamine (44), for which an electronic absorption spectrum with A.max = 689 nm was observed, the main reaction route is the formation of a benzidine-type dimer, similarly to that in the case of 39+. The dimerization rate constant is 2.3 x 104 M 1 s 1. However, in the presence of a large excess of parent molecules acting as a base, the formation of cyclized dimers was also suggested. In contrast, the formation of cyclic structures was found to be characteristic of the radical cations of 3 -substituted (45) and 3,3 -disubstituted (46) derivatives of diphenylamine. On the basis of CV measurements, the formation of dihydrodiphenylphenazine derivatives may be anticipated and, consequently, 47 is assumed to be the product of dimerization of 45. In both cases the rate constants were as fast as ca 1 x 107 M 1 s-1 the 3-methyl substituent promotes visibly the reaction between the 6-position of the phenyl ring of... 

The value of generating a reactive species by several independent routes is well illustrated in the case of the phenyl radical. Very recently, a thorough matrix-isolation and theoretical study has been made of the electronic absorption spectrum and IR spectrum of Ph-, in which the radical was generated by photolysis (193 or 248 nm) of phenyl iodide and other precursors, as well as by thermolysis reactions. Several D-substituted isotopomers of the radical were also investigated, and polarized light was used to... 

Radziszewski, J.G., Electronic absorption spectrum of phenyl radical, Chem. Phys. Lett., 301,565,1999. 

The electronic absorption spectra of the products of one-electron electrochemical reduction of the iron(III) phenyl porphyrin complexes have characteristics of both iron(II) porphyrin and iron(III) porphyrin radical anion species, and an electronic structure involving both re.sonance forms Fe"(Por)Ph] and tFe "(Por—)Ph has been propo.sed. Chemical reduction of Fe(TPP)R to the iron(II) anion Fe(TPP)R) (R = Et or /7-Pr) was achieved using Li BHEt3 or K(BH(i-Bu)3 as the reductant in benzene/THF solution at room temperature in the dark. The resonances of the -propyl group in the F NMR spectrum of Fe(TPP)(rt-Pr) appear in the upfield positions (—0.5 to —6.0 ppm) expected for a diamagnetic porphyrin complex. This contrasts with the paramagnetic, 5 = 2 spin state observed... 

Ito and Matsuda studied the y-radiolysis of 2-methyltetrahydrofuran (MTHF) solutions of diphenyl sulfone and dibenzothiophene-S,S-dioxide (DBTSD) at 77 K. They found that the radical anions of these sulfone compounds are formed and have intense absorption bands at 1030 nm and 850 nm, respectively. The blue glassy solution of y-irradiated diphenyl sulfone has absorption bands at both 1030 nm and 360 nm while the absorption spectrum of the benzenesulfonyl radical formed by UV irradiation of diphenyl sulfone solution at 77 K showed only a peak at 382 nm. Gamma-irradiated phenyl methyl sulfone solution showed an absorption band only at 385 nm. Consequently the appearance of the absorption bands in 800-1030 nm of diphenyl sulfone and DBTSD may suggest that the unpaired electron is delocalized on two phenyl rings. The same authors studied the radiolysis of MTHF solutions of disulfones (diphenyl and dihexyl disulfones). They found a blue coloring of the solution by the y-radiolysis of diphenyl disulfone and dihexyl disulfone due to absorption peaks at 695 nm and 690 nm respectively, besides smaller absorptions at 300-400 nm. Comparing these results to the previous observation, that phenyl methyl sulfone solution absorbs only at 398 nm, results in the conclusion that the absorption band at 690 nm is due to the linked two sulfone moieties. The authors found that substituents on the phenyl ring lead to shifts in the absorption maxima of the... 

The delocalization of the conduction electron onto the side chains would be expected if the pendant groups were replaced with more electrophilic substituents than the phenyl group. However, this is not the case. Figure 22 shows the absorption spectrum of poly-(methylnaphthylsilane) radical anion. The absorption spectrum is very similar to that of the naphthalene radical anion, which implies that the unpaired electron is localized on the pendant group. Increase of the electron affinity of pendant groups does not necessarily cause the delocalization. 

Pulse radiolysis has been used to afford the radical cations of thioanisole, p-methylthioanisole, and benzyl phenyl sulfides. The absorption spectra of these radicals and their reaction with a number of nucleophiles and electron donor s are reported.302 Exposure of trimethylphosphine sulfide to 60Co 7-radiation at 77 K gave the radical anion Me3PS , identified by its ESR spectrum, from which other radicals, including Me2PS, were formed at higher temperatures.303... 

Porter and Ward (1965) have obtained the optical absorption spectrum of the phenyl radical by flash photolysis and from an analysis of the wavelength shifts observed for a series of halogenated phenyl radicals they conclude that the ground state of the phenyl radical corresponds to the first structure (i.e. the unpaired electron remains in the sp -orbital). Thus it was of particular interest to obtain the e.s.r. spectrum of the phenyl radical for this would give additional information and help to assign the correct structure (Bennett et al., 1966a). 

The electronic absorption spectmm of phenyl radical is an interesting example where computational approaches can be compared to the experimental spectrum assigned to the above-mentioned radical on the basis of the strict correlation of intensity evolution in simultaneously measured IR and UV-vis spectra, for which several independent precursors gave consistent results. 

Dissociation constants of 2 have been reported to be higher for dimers with electron donating substituents in the para positions of the phenyl rings [ 15]. We found that these groups in ortho positions resulted qualitatively in very different properties. Any ortho substituent in Ar decreased dissociation. Thus, a benzene solution of the stable dimer of 2-(2-chlorophenyl)-4,5-diphenylimidazolyl radical 2d exhibited no radical absorption spectrum below 80°, whereas the 4-chloro compound 2e was partially dissociated in solution at room temperature. Ortho substituents in Ar and Ar" affected the equilibrium quite modestly but appeared to have the opposite effect, that is, increased dissociation. Qualitatively, meta substituents had little effect on the equilibrium of dissociation. 

Scaiano and Kim-Thuan (1983) searched without success for the electronic spectrum of the phenyl cation using laser techniques. Ambroz et al. (1980) photolysed solutions of three arenediazonium salts in a glass matrix of 3 M LiCl in 1 1 (v/v) water/acetone at 77 K. With 2,4,5-trimethoxybenzenediazonium hexafluorophos-phate Ambroz et al. observed two relatively weak absorption bands at 415 and 442 nm (no e-values given) and a reduction in the intensity of the 370 nm band of the diazonium ion. The absence of any ESR signals indicates that these new bands are not due to aryl radicals, but to the aryl cation in its triplet ground state. 

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