Photoionization anions

ESI operating in the negative ion (Nl) mode has been the interface most widely used for the analysis of anionic PFCs. In addition, ESI has also been optimized for the determination of neutral compounds, such as the sulphonamides PFOSA, Et-PFOSA, and t-Bu-PFOS. The use of atmospheric pressure photoionization (APPI) has been explored by Takino and collaborators [88]. The authors found the main advantage of this technology to be the absence of matrix effects, but the limits of detection were considerably higher than those obtained by LC-ESTMS/MS. 

Matrix-isolated alkali atoms (or small clusters) also undergo easy photoionization, and the electrons released in this process may attach themselves to nearby substrates to form the corresponding radical anions. However, one drawback of alkah metal atoms or clusters is that they tend to swamp the electronic absorption spectrum of the target reactive intermediate that can only thus be detected by IR. 

Another approach for the formation of radical anions by LFP has been developed to overcome some of these difficulties. The approach involves the formation of radical anions by trapping a solvated electron produced by photoionization of 4,4 -dimethoxystilbene (DMS) to its cation radical (equations 31 and 32). This photoionization/electron trapping method is quite general for substrates that are transparent where DMS absorbs and that are more easily reduced than dimethoxystilbene. In many ways, this method is similar to pulse radiolysis, another useful approach used to generate radical anions for optical kinetic studies. 

Upon ejection from an ion or molecule by photoionization or high energy radiolysis, the electron can be captured in the solvent to form an anionic species. This species is called the solvated electron and has properties reminiscent of molecular anions redox potential of —2.75eV and diffusion coefficient of 4.5 x 10-9 m2 s-1 (Hart and Anbar [17]) in water. Reactions between this very strong reductant and an oxidising agent are usually very fast. The agreement between experimental results and the Smoluchowski theoretical rate coefficients [3] is often close and within experimental error. For instance, the rate coefficient for reaction of the solvated (hydrated) electron in water with nitrobenzene has a value 3.3 x 10+1° dm3 mol-1 s-1. 

Prof. Schlag ( ZEKE Spectroscopy, this volume) has introduced a new sequential technique of ZEKE spectroscopy In the first step, a negative ion M is photoionized, yielding the neutral core M of the excited Rydberg state of the anion M. In the second step, M is further photoionized, yielding the cationic core M+ of the excited Rydberg state of the neutral molecule M. The overall sequence is thus... 

Mathivanan, Johnston and Wayner examined the effect of substituents on the rates of cleavage of a-phenoxyacetophenone radical anions (equation 6)43. In this study, the radical anions were generated by trapping solvated electrons produced by laser-induced photoionization of 4,4 -dimethoxystilbene in CH3CN and DMF. Their results show that the fragmentation rate is enhanced when Y is electron withdrawing, but retarded when X is electron withdrawing. 

Sometimes, anion radicals are formed indirectly, by means of special chemical reactions. Photoionization of hydrazine in a mixture of liquid ammonia with THF in the presence of potassium l-butoxide leads to the formation of the diazene anion radical by the sequence of the following reactions (Brand et al. 1985) ... 

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 of doped fullerene anions Coulomb confinement resonances 38... 

Figure 9 The A-potential model calculated results [28] for the Is photoionization of free Ne, as well as that of Ne from neutral Ne C6o (z = 0) and from the Ne C 0 anion with z = -1, -2, -3, and -5 (dotted line), as indicated.

Figure 11 Nondipole photoelectron angular asymmetry parameter y A ( >) for Is photoionization of free Ne, Ne from neutral Ne Cgo as well as Ne from the fullerene anion Ne q0 (z < 0) [28], as indicated.

Photoionization is also an important (although not the major) decay pathway for the excited diphenylketyl radical (161) in polar solvents such as acetonitrile [137] (Scheme 22). This was confirmed with the observation of benzophenone radical anion in laser flash experiments following 515-nm photolysis of ground-state 161. The anion radical is apparently produced as a result of electron ejection followed by trapping of the electron by ground-state benzophenone. 

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). 

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