Pyridinyl radical absorption

Fig. 10. Absorption spectra of four l-alkyl-4-carbomethoxy-pyridinyl radicals, (R), in thin films at 77 K. The three main bands are at 6S0 nm, 400 nm and 300 nm. The 650 nm band (charge-transfer absorption of pimer) shifts from 623 nm for I-CH3 to 675 nm for l-lCHsfsC, along with a two-fold intensity decrease. The 400 nm band is composed of the pyridinyl radical absorption of the pimer, and absorptions of the two ester conformational isomers of the pyridinyl radical monomer. The 300 nm band varies with the different relative contributions of the pimer and the monomer...

The copper(I) alkynyls displayed rich photochemistry and particularly strong photoreducing properties. The transient absorption difference spectrum of [Cu3(dppm)3(/X3-) -C=CPh)2]+ and the electron acceptor 4-(methoxycarbonyl)-A-methylpyridinium ion showed an intense characteristic pyridinyl radical absorption band at ca. 400 nm. An additional broad near-infrared absorption band was also observed and it was assigned as an intervalence-transfer transition of the mixed-valence transient species [Cu Cu Cu (dppm)3(/x3- -C=CPh)2] +. The interesting photophysical and photochemical properties of other copper(I) alkynyl complexes such as [Cu(BTA)(hfac)], 2 [Cui6(hfac)8(C=C Bu)8], and [Cn2o(hfac)8(CsCCH2Ph)i2] have also been studied. 

The photoinduced electron transfer reactions have also been studied with nanosecond transient absorption spectroscopy [67]. The transient absorption difference spectrum for the reaction of 12a and 4-(methoxycarbonyl)-A/-methylpyr-idinium is shown in Fig. 4. The difference spectrum is characterized by a sharp intense absorption at approximately 390 nm, a Iowa- intensity band at 484 nm, and an intense broad absorption band at approximately 693 nm. The sharp band at around 390 nm is characteristic of pyridinyl radical absorption. The reaction mechanism is depicted in Scheme 1. 

Various compounds were shown to sensitize the photochemical decomposition of pyridinium salts. Photolysis of pyridinium salts in the presence of sensitizers such as anthracene, perylene and phenothiazine proceeds by an electron transfer from the excited state sensitizer to the pyridinium salt. Thus, a sensitizer radical cation and pyridinyl radical are formed as shown for the case of anthracene in Scheme 15. The latter rapidly decomposes to give pyridine and an ethoxy radical. Evidence for the proposed mechanism was obtained by observation of the absorption spectra of relevant radical cations upon laser flash photolysis of methylene chloride solutions containing sensitizers and pyridinium salt [64]. Moreover, estimates of the free energy change by the Rehm-Weller equation [65] give highly favorable values for anthracene, perylene, phenothiazine and thioxanthone sensitized systems, whilst benzophenone and acetophenone seemed not to be suitable sensitizers (Table 5). The failure of the polymerization experiments sensitized by benzophenone and acetophenone in the absence of a hydrogen donor is consistent with the proposed electron transfer mechanism. 

Exhaustive reduction of an exactly known amount of pyridinium salt (weight, absorption spectrum) leads to accurate absorption coefficients which agree with those obtained by alternative means. Electrochemical techniques have been used to probe some of the chemical properties of both mono- and bis-pyridinyl radicals (see also Sect. 4.5). 

The refractive index of the thin film may be estimated from the amplitude of the interferometric signal with Eq. 9, starting from the" first maximum, and provided that neither absorption nor dispersion affect the signal intensity too greatly. A refractive index of 1.50 for the pyridinyl radicals at 77 K has been verified. 

Pyridinyl radicals and derived species exhibit characteristic absorption spectra, especially in the near ultraviolet and visible regions. These spectra have been useful in identifying the radicals, in decisions about the nature of the species present under given conditions, in following the kinetics of reactions, and in revealing many subtle features of pyridinyl radical behavior. 

Pyridinyl radical monomers are weakly colored in dilute solutions, with a weak absorption band in the visible or near IR (640-1250 nm) and two strong absorption bands (365-410 nm 280-309 nm) in the in the near UV (Table 1). A fourth absorption of moderate intensity near 225 nm has been observed for a number of pyridinyl radicals. 

Table 1. Absorption Maxima in nm for Pyridinyl Radicals in Solution...

The strong and striking blue color of distilled pyridinyl radicals is due to a charge-transfer absorption in a pyridinyl-pyridinyl complex (a pimer) (Eq. 10), first identified by Itoh and Nagakura The electronic transition may be described by Eq. 11. 

The N-methyl stretch absorptions are often weaker in methyl groups attached to electronegative atoms . Spin density on the nitrogen in the pyridinyl radical reflects a positive charge dimerization of the radical would diminish the transfer of charger into another group, as whon with resonance forms (see below). 

Fig. 22. Spectroscopic changes on warming l-methyl-4-carbo-t-butoxypyridinyl radical. Pimer absorption is tost completely brfore loss of all radical absorption in the 400 nm region, and parallels the decrease in the 380 nm absorption. At —65 °C, very little pyridinyl radical is present. The long wavelength absorption (rar wj transition in the dimer) extends from 350 nm past 500 nm...

The basicity of pyridinyl radicals is of interest since some pyridinyl radicals react with one another in an acid catalyzed reaction (sect. 4.4). The absorption spectra of radicals generated from the pyridinium ion by pulse radiolysis in aqueous solution at different pH values allow the evaluation of the basicity of the radicals 97,98) I jjg Qf protonated form of l-methyl-3-carbamidopyridinyI radical, (CONHj) ), is 1.43 the protonated radical has alsorption maxima at 3(X) nm and 440 nm, at somewhat longer wavelengths than those for the unprotonated radical at 280 nm and 420 nm. The nicotinamide adenine dinucleotide radical (NAD ) has a pK, of 0 or less, with a shift in absorption maximum due to protonation being observed only in 1.5 M HCIO4. 

The pK, of the protonated (CONH2)4 radical is 2.0, with a shift from 425 nm to 400 nm on deprotonarion. The change in the absorption maxima for both 3- and 4-carbamidopyridinyl radicals implies protonation on the amide group rather than the ring (Eq. 26). Thus, the pyridinyl radicals are more basic than benzamide derivatives, but very weakly basic compared to dihydropyridines (pK, ca. 7) or typical tertiary amin (pK, ca. 10-11). 

Fig. S. The 400-nm absorption band is characteristic of the pyridinyl radical. The reaction involves the reduction of the pyridinium cation to the pyridinyl radical, while the 24a is oxidized to the mixed-valence species [Cu(I)Cu(I)Cu(II)] . ...

Moreover, when pyridinium or quinolinium is used as the cationic partner of the carbonyl metallates, additional absorption bands (at shorter wavelengths) are observed which are readily assigned to the pyridinyl [171] and quinolinyl [172] radicals, respectively (see Figure 13). 

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