Radical cations photoionization

For many years, investigations on the electronic structure of organic radical cations in general, and of polyenes in particular, were dominated by PE spectroscopy which represented by far the most copious source of data on this subject. Consequently, attention was focussed mainly on those excited states of radical ions which can be formed by direct photoionization. However, promotion of electrons into virtual MOs of radical cations is also possible, but as the corresponding excited states cannot be attained by a one-photon process from the neutral molecule they do not manifest themselves in PE spectra. On the other hand, they can be reached by electronic excitation of the radical cations, provided that the corresponding transitions are allowed by electric-dipole selection rules. As will be shown in Section III.C, the description of such states requires an extension of the simple models used in Section n, but before going into this, we would like to discuss them in a qualitative way and give a brief account of experimental techniques used to study them. 

Radical cations can be formed by irradiation of unsubstttuted aromatic hydrocarbons such as naphthalene, and this makes possible the photochemical displacement of hydride ion by a nucleophile such ascyanide f3.10). Oxygen is not necessary for the success of this type of reaction if a good electron-acceptor is present, such as p-dicyanobenzene (3.11), which enhances the initial photoionization and also provides for reaction with the displaced hydrogen. 

The photooxidation of p-phenylenediamine to the Wurster s Blue radical cation apparently proceeds by photoionization of the excited triplet state of the neutral molecule,219 and it has been suggested that the delayed fluorescence of perylene may be partly due to photoionization of its triplet state and slow subsequent recombination of... 

When a photon of sufficient energy impinges on a neutral molecule, M, it is possible to eject an electron, leaving behind a radical cation, M in one of several possible electronic states. It is well-known that energy is conserved during the photoionization event hence if hv represents the energy of the incident photons in Eq. 1 it is obvious that measurement of the kinetic energies of the... 

In the direct effect of ionizing radiation on DNA, radical cations are the primary products (Chap. 12). For this reason, their reactions are of considerable interest. Obviously, photoionization (e.g., at 193 nm) and laser multi-photon excitation leads to such species (e.g., Candeias and Steenken 1992b Malone et al. 1995 Chap. 2.2). Base radical cation electron pairs have been proposed to be the first observable intermediates with a lifetime of 10 ps for Ade and four times longer for the other nucleobases (Reuther et al. 2000). Radical cations are also assumed to be intermediates in the reactions of photosensitization reactions with qui-nones, benzophenone, phthalocyanine and riboflavin (Cadet et al. 1983a Decar-roz et al. 1987 Krishna et al. 1987 Ravanat et al. 1991, 1992 Buchko et al. 1993 Douki and Cadet 1999 Ma et al. 2000). Nucleobase radical cations may be produced by electrochemical oxidation (Nishimoto et al. 1992 Hatta et al. 2001) or with strongly oxidizing radicals (for a compilation of their reduction potentials see Chap. 5.3). Rate constants are compiled in Table 10.3. 

This view is been confirmed by an electrochemical product study (Hatta et al. 2001) that is discussed below. The pfCa value of the Thy radical cation has been determined at 3.2 (Geimer and Beckert 1998). When the position at N( ) is substituted by a methyl group and deprotonation of the radical cation can no longer occur at this position, deprotonation occurs at N(3) (Geimer and Beckert 1999 for spin density calculations using density functional theory (DFT) see Naumov et al. 2000). This N(3) type radical is also produced upon biphotonic photoionization of N(l)-substituted Thy anions [reaction (7)] in basic 8 molar NaC104 D20 glasses which allowed to measure their EPR spectra under such conditions (Sevilla 1976). 

The radical anions may be formed by reacting the nucleobases with eaq which may be either generated radiolytically or in a two-step reaction, e.g in the laser flash photolysis of anthraquinonedisulfonate in the presence of pyrimidines (yielding the pyrimidine radical cation and an anthraquinonedisulfonate radical anion) and subsequent photoionization of the anthraquinonedisulfonate radical anion (Lii et al. 2001). The latter approach, combined with Fourier transform EPR spectroscopy, yielded detailed information as to the conformation of the radical anions of Ura and Thy in aqueous solution (for a discussion see Close 2002 Naumov and Beckert 2002). Similarly valuable EPR information has been obtained from y-irradiated single crystals (cf. Box and Budzinski 1975 Boxet al. 1975 Sagstuen et al. 1998). 

Photoionization at 193 nm in oxygenated solution of poly(C) causes strand breakage with high efficiency, half of which occurs at times < 4 ms, the other half with a half-life of 7 ms (Melvin et al. 1996 Table 11.8). This kinetic behavior is very different from what is seen after -OH-attack and points to the direct involvement of the Cyt radical cation. In poly(U), the (biphotonic) photoionization shows similar results (Table 11.8). With poly(A), the formation of strand breaks is 20-times less efficient as compared to poly(C) (Table 11.8), and this is in agreement with the above conclusion that the A-+ or A- do not cause strand breakage to any major extent. 

As with other reducing agents, G reacts with 02 (Chap. 10.2) which is the most abundant freely diffusing peroxyl radical. Upon two-photon excitation of a 2-aminopurine-containing ss- and dsODN in air-saturated solutions, photoionization leads to the formation of eaq (and subsequently 02 ) and the 2-amino-purine radical cation oxidizes a neighboring G (leading to G plus H+ Misiaszek et al. 2004). G and 02 react with one another (ssDNA k = 4.1 x 108 dm3 mol-1 s 1 dsDNA 2.7 x 108 dm3 mol-1 sH). In the majority of these events G is reformed, but with an efficiency of 15% Iz and, to a minor extent, 8-oxo-G are formed. The suggested mechanism is shown in Chapter 10.2. 

Photoelectron spectroscopy uses the photoionization of a neutral molecule M to its radical cation M ,... 

Known to be a direct process by electrochemical evidence. c Probably initiated by photoionization to give a radical cation (Letsinger and McCain, 1966). Homolytic via CN ... 

Coal and many coal-derived liquids contain polycyclic aromatic structures, whose molecular equivalents form radical cations at anodes and radical anions at cathodes. ESR-electrolysis experiments support this (14). Chemically, radical cations form by action of H2SO4 (15,19), acidic media containing oxidizing agents (15,20,21,22), Lewis acid media (18,23-35) halogens (36), iodine and AgC104 (37,38), and metal salts (39,40). They also form by photoionization (41,42,43) and on such solid catalytic surfaces as gamma-alumina (44), silica-alumina (45), and zeolites (46). Radical anions form in the presence of active metals (76). 

The primary steps of the photolysis of aqueous monuron and diuron were investigated by Canle et al. by means of transient absorption spectroscopy using an ArF laser (X = 193 nm) for excitation [89]. Under these conditions, photoionization occurred with a quantum yield of about 10%. Radical cations were detected after the laser pulse and found to deprotonate to yield neutral radicals [89]. 

The mass spectra obtained by the APPI source in the positive ion mode are characterized by the presence of two main types of ions of the molecular species that may coexist [82] the radical cation M + and the protonated molecule [M + H]+. In direct APPI, the reaction is the classical photoionization leading to the radical cation of the molecular species ... 

For example, Vincow and coworkers (205) have prepared radical cations of benzene, hexamethylbenzene, perylene, naphthalene, etc., by photoionization of the molecules in a rigid glass, while Hulme and Symons (206) have... 

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