Absorption spectra, of free radicals

C. G. Chatgilialoglu and M. Guerra,/. Am. Chem. Soc., 112, 2854 (1990). MSXa Study of Absorption Spectra of Free Radicals. Characterization of Rydberg and Valence Transitions in Alkyl Derivatives of Group 14 Centered Radicals. 

One of the most powerful techniques for studying the absorption spectra of free radicals, particularly polyatomic species, is that of flash photolysis. A summary of the polyatomic species which have been studied spectroscopically (3) is given in Table II approximately one-half of these have been found by flash photolysis. In this technique a parent molecule is decomposed by an intense light flash... 

It is only very recently that attempts to obtain infrared spectra of free radicals have been successful.2 As these infrared studies are further developed, they promise to fill many of the gaps left by ultraviolet investigations, both with regard to radicals studied (since all radicals must have a discrete infrared spectrum), and with regard to the fundamental frequencies of the ground state, which in most cases are difficult to obtain from ultraviolet absorption spectra. 

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. 

The experimental methods available for the absorption spectroscopy of free radicals are critically discussed with special reference to flash photolysis and spectroscopy. Some free radical spectra which have been obtained in this way are described and values are given for the dissociation energies and vibration frequencies in the upper and lower states of the CIO, SH and SD radicals. 

Matrix-Isolated NH2. Absorption spectra of NH2 radicals, which were produced in a microwave discharge of Ar mixed with a small amount of NH3 or N2H4 and trapped on a 4.2 K surface [30, 31], indicated nearly free rotation of NH2 in an Ar matrix [32] for frequencies of assigned absorption bands, see Table 14. 

Chemical methods for structure determination in diene pol3 mers have in large measure been superseded by infrared absorption techniques. By comparing the infrared absorption spectra of polybutadiene and of the olefins chosen as models whose ethylenic structures correspond to the respective structural units, it has been possible to show that the bands occurring at 910.5, 966.5, and 724 cm. are characteristic of the 1,2, the mns-1,4, and the m-1,4 units, respectively. Moreover, the proportion of each unit may be determined within 1 or 2 percent from measurements of the absorption intensity in each band. The extinction coefficients characteristic of each structure must, of course, be known these may be assigned from intensity measurements on model compounds. Since the proportions of the various units depend on the rates of competitive reactions, their percentages may be expected to vary with the polymerization temperature. The 1,2 unit occurs to the extent of 18 to 22 percent of the total, almost independent of the temperature, in free-radical-polymerized (emulsion or mass) poly butadiene. The ratio of trans-1,4 to cfs-1,4, however,... 

Closs and Trifunac, 1970 Baldwin and Andrist, 1971 Lepley and Closs, 1972 Bethell and McDonald, 1977). The formation of free radicals from aromatic carbenes is often easily detected by the fast laser spectroscopic techniques discussed earlier. The radicals generally have characteristic absorption spectra and reactivity patterns that make their identification certain. The direct insertion reaction of singlet carbenes is not expected to generate free radicals. 

In appearance, the PHIP phenomenon closely resembles those due to CIDNP [16], another phenomenon, which also gives rise to emission and enhanced absorption lines in NMR spectra. However, CIDNP is the consequence of the occurrence of free radicals, and previously has frequently been considered unequivocal proof for free radical reactions. 

The redox properties elicited for Rh(bpy)3 + and its congeners are thus entirely consistent with the description of these species as bound-ligand radicals. On the other hand, the disproportionation reactions eq 2-6 are not known to be characteristic of ligand-centered radicals, but are consistent with behavior expected for rhodium(II). Furthermore the substitution lability deduced for Rh(bpy)3 + and Rh(bpy)2 +> while consistent with that expected for Rh(II), is orders of magnitude too great for Rh(lII). Finally the spectrum observed for the intermediate Rh(bpy)3 + is not that expected for [RhIII(bpy)2(bpy")]2+. The spectrum measured has an absorption maximum at 350 nm with e 4 x 10 M 1 cm l and a broad maximum at 500 nm with e = 1 x 1()3 M 1 cm l. The spectra of free and bound bpy radical anions are quite distinctive (23.35-38) very intense absorption maxima (e 1 x 10 to 4 x 10 M - cm l) are found at 350-390 nm and are accompanied by less intense maxima (e 5 x 10 cm ) at 400 to 600 nm. While the Rh(bpy)3 +... 

This technique for the study of a fast reaction is gas phase or liquid phase was developed by Norrish and Poster. This is an example of Pulse method which initiates a reaction by creating new reactive species—excited electronic states, radicals, ions in the system under study. The method uses a light flash of high intensity for a very short duration (10- s) to produce atoms or free radicals or excited species in a system. These are at a fairly high concentration and undergo further reactions which are followed spectroscopically. A spectroscopic flash of light is followed by the initial flash by some fraction of a millisecond. The absorption spectra of all the species that are formed within the system can be recorded. One cannot only get indications of what species are formed but also how these species give rise to others. Thus a very direct picture of the kinetic behaviour of a fast reaction can be obtained. 

Absorption of sunlight induces photochemistry and generates a variety of free radicals that drive the chemistry of the troposphere as well as the stratosphere. This chapter focuses on the absorption spectra and photochemistry of important atmospheric species. These data can be used in conjunction with the actinic fluxes described in the preceding chapter to estimate rates of photolysis of various molecules as well as the rate of generation of photolysis products, including free radicals, from these photochemical processes. 

Figures 4.26, 4.27, and 4.28 show typical UV absorption spectra for some simple aldehydes and ketones (Rogers, 1990 Martinez et al., 1992 see also Cronin and Zhu, 1998, for n-pentanal). Formaldehyde stands out from the higher aldehydes and ketones in that it has a highly structured spectrum and furthermore, the absorption extends out to longer wavelengths. The latter difference is particularly important because the solar intensity increases rapidly with wavelength here (Chapter 3.C.1) and hence the photolysis rate constant for HCHO and the rate of production of free radicals...

Figures 4.35, 4.36, and 4.37 show the absorption spectra of the free radicals CIO, BrO, and IO, respectively (Wahner et al., 1988 DeMore et al., 1997 Laszlo et al., 1995). All have beautifully banded structures at longer wavelengths and large absorption cross sections, which allows one to measure these species in laboratory and atmospheric systems using differential optical absorption spectrometery (DOAS) (see Chapter 11.A.Id). However, as in the case of HCHO, adequate resolution is an important factor in obtaining accurate cross sections.

The reaction cell has a White cell optical system (see Chapter ll.A.lc) with a pulsed xenon lamp light source. Once the radicals are formed, they are detected by their absorptions in the UV using the Xe lamp and a monochromator-photomultiplier or photodiode array detector. Thus the absorption spectra of the free radicals generated in the system can be measured and the absorption at a particular wavelength used to follow their reaction kinetics. 

Radical anions are produced in a number of ways from suitable reducing agents. Common methods of generation of radical anions using LFP involve photoinduced electron transfer (PET) by irradiation of donor-acceptor charge transfer complexes (equation 28) or by photoexcitation of a sensitizer substrate (S) in the presence of a suitable donor/acceptor partner (equations 29 and 30). Both techniques result in the formation of a cation radical/radical anion pair. Often the difficulty of overlapping absorption spectra of the cation radical and radical anion hinders detection of the radical anion by optical methods. Another complication in these methods is the efficient back electron transfer in the geminate cation radical/radical anion pair initially formed on ET, which often results in low yields of the free ions. In addition, direct irradiation of a substrate of interest often results in efficient photochemical processes from the excited state (S ) that compete with PET. 

Almost all spectroscopic studies of free radicals have been carried out in the visible and ultraviolet regions, and as a consequence only those radicals that have discrete spectra in these regions have been amenable to detailed investigation. Our inability to find any spectroscopic evidence for certain radicals, such as BH3, C2H, CF3, and others, in spite of their almost certain presence under the conditions of our experiments, is probably due to the fact that these radicals have no stable excited states, and therefore give rise only to (weak) continuous absorptions, which are difficult to detect and identify. (If CH4 were a free radical, it would be exceedingly difficult to detect spectroscopically, since it has only continuous absorptions except, of course, in the infrared.)... 

Spectroscopic observation of free ions [48] The absorption spectra of many aromatic radical ions have been determined in rigid matrices, where they are formed by ionization reactions (X-ray or y-ray irradiation). On the assumption that these spectra remain practically unchanged in the liquid state, flash photolysis can then be used to measure the ion yields in various solvents. The major problem is that the exact values of the extinction coefficients are seldom known, and this introduces some uncertainty on the absolute ion yields the relative yields are, however, reliable. 

Appropriate modifications of the ESR spectrometer and generation of free radicals by flash photolysis allow time-resolved (TR) ESR spectroscopy [71]. Spectra observed under these conditions are remarkable for their signal directions and intensities. They may be enhanced as much as one hundredfold and may appear in absorption, emission, or in a combination of both modes. These spectra indicate the intermediacy of radicals with substantial deviations from equilibrium populations. Significantly, the splitting pattern characteristic for the spin density distribution of the intermediate remains unaffected thus, the CIDEP (chemically induced dynamic electron polarization) enhancement facilitates the detection of short-lived radicals at low concentrations. 

In this account, we will focus on the transient analysis of these systems, which has strongly contributed to a deeper understanding of the diverse reaction modes (Patemo-Buchi, proton abstraction, cycloaddition). In general, aromatic ketones were selected as electron acceptors for reasons of suitable excitation and long wavelength absorption of the radical anion intermediates. Among them, fluorenone 3 is particularly well suited since the concentration, solvent, temperature, and cation radius dependence of the absorption spectra of pairs formed with metal cations are already known [29]. Hogen-Esch and Smid [30, 10] pointed out that a differentiation between CIP and SSIP is possible for fluorenone systems. On the other hand, FRI s and SSIP s cannot be differentiated simply by their UV/Vis absorption spectra, whereas for instance conductance measurements may be successful. However, the portion of free radical ions in fluorenyl salt solutions was shown to be less important [9, 31]... 

Pristine SWCNTs and their fluorinated derivatives, F-SWCNTs, were reacted with organic peroxides to functionalize their sidewalls covalently by attachment of free radicals (Scheme 1.15). The tubes reactivity towards radical addition was compared with that of corresponding polyaromatic and conjugated polyene JT-systems [150, 151]. The characterization of the functionalized SWCNTs and F-SWCNTs was performed by Raman, FT-IR and UV/Vis/NIR spectroscopy and also by TGA/MS, TGA/FT-IR and with TEM measurements. The solution-phase UV/Vis/NIR spectra showed complete loss of the van Hove absorption band structure, typical of functionalized SWCNTs [150]. 

Photoinitiators are perhaps the most important component in uv cured radiation coatings. The photoinitiator is an ingredient that absorbs light and is responsible for the production of free radicals in a free radical polymerized system or cations in a cationic photoinitiated system. The photoinitiators are usually added to the reactive coating formulations in concentration ranges from less than 1 to 20 percent by weight based on the total formulation. The absorption bands of the photoinitiators should overlap the emission spectra of the various commercial light sources. 

---