Radical anions hyperfine structure

In a number of cases quantum-chemical calculations have confirmed the experimentally determined distribution of unpaired electron density in a radical ion [94]. In addition the hyperfine structure of the spectrum can serve as an experimental basis for verification and confirmation of the validity of complex quantum-chemical calculations applied to the structure of organic molecules and radicals. The hyperfine structure can also serve to identify a given type of radical, and to offer information on the coplanarity and isomerism of radical anions it can also be used to determine the rates of transi-... 

Because of the inherent non-planar structure of helicenes it seemed of interest to examine the spin distribution in helicene radical anions. For the mono anion of hexahelicene a set of 8 hyperfine splitting constants (hfsc s) and 38 = 6561 ESR lines can be expected. Such a spectrum will be poorly resolved. Indeed, it was not possible to determine hfsc s from the ESR-spectrum of hexahelicene 132). Using the ENDOR technique which reduces the amount of lines the eight hfsc s could be deduced, however, and the relative signs could be determined l33) by the triple resonance technique. 

Early ESR studies demonstrated that the hyperfine coupling constant (ac 13) for 13C(car-bonyl)-substituted fluorenone radical anion is counterion-dependent. For the free ion, ac 13 = 2.75 Gauss. In contrast, when the counterion is Li+, ac 13 = 6.2 Gauss23. Consider Scheme 4 For the free ion, canonical structure 1 and 2 are contributors to the resonance hybrid. For the >C=0 / Li+ ion pair, association of Li+ with oxygen increases the relative contribution of canonical structure 1 to the resonance hybrid, resulting in greater spin density at carbon. The fact that spin (and charge density) varies as a function of counterion (and presumably solvent) will certainly affect the reactivity of the radical ion. However, very few quantitative studies exist which directly address this point. 

Occasionally such intermolecular effects are specific and have sufficient lifetime to contribute individually to the spectrum. This is the case, for example, when organic radical-anions are studied in solvents of low ionizing power such as tetrahydrofuran. Ion-pairing then becomes important and, when alkali-metal cations are involved, the effect on the spectrum is to split each hyperfine component into four lines, each having one-quarter of the original intensity. This may convert a complicated spectrum into one that is quite uninterpretable, and can be avoided by using a better solvent or non-magnetic cation. However, it also provides evidence that contact ion-pairs are important in such solutions, and yields structural details unobtainable by other techniques. 

The ESR spectra of the anion radicals 246 derived from a series of 4-dicyanom ethyl idene-4//-thiopy ran sulfones show clearly resolved hyperfine structure centred around g values of ca. 2000. Large nitrogen splitting of 1.0-1.5 mT and FI-3 and FI-5 splitting of 0.3-0.93 mT are observed <1995JOC1674>. 

The radical ions of CF2=CF2 well illustrate the fact that the geometry and the electronic structure of a radical cation can be markedly different from those of the corresponding radical anion. As mentioned above (CF2=CF2) adopts a chair structure 18 (C2h)30, while (CF2=CF2)+ produced by irradiation of a dilute solution of C2F4 in FCC13 at 77 K is characterized by ESR parameters (Table 28) which are consistent with a planar structure with the unpaired electron in a bonding n-orbital283,284. An extra splitting was attributed to hyperfine interaction with a fluorine of the matrix. 

Nitro derivatives of azoles owing to electron deficiency are capable of being reduced with the formation of radical ions. 3-Nitropyrazole in the conditions of pulse radiolysis, depending on pH medium, forms radical anions (RA) or radical dianions (RDA), which were registered by ESR method [849] (Scheme 3.16). A hyperfine structure (HFS) constant (a, mT) is a main parameter in the ESR spectrum indicating the interaction of the unpaired electron with all magnetic active nuclei of radical. 

The reduction in the hyperfine splitting is attributed to the participation of the structure (CH3)2CO- in the ketyl radical anion which allows delocalization of the unpaired electron to the oxygen atom (Symons, 1963). 

In the present experiment, we are concerned with the hyperfine structure of the ben-zosemiquinone radical anions. The delocalized unpaired tt electron is of course distributed over the entire molecular frame of six C atoms and two O atoms. With R = H, by symmetry, it is clear that the four protons are all equivalent in the para species hence five hyperfine lines with relative intensities 1 4 6 4 1 are expected in the ESR spectrum of this radical. By contrast, when R is not a proton, the three ring protons are not related by symmetry, and thus each may be expected to possess a different splitting constant. A hyperfine structure pattern of eight unequally spaced lines of equal intensity is expected. The line splittings and relative intensities in ESR spectra thus convey information about the geometric arrangement of the atoms. 

Group 3 (Sc, Y, La) metallofullerenes exhibit ESR hfs, which provides us with important information on the electronic structures of the metallofullerenes. Typical ESR-active monometallofullerenes are La Cs2, Y Cs2, and Sc C82- The ESR hfs of a metallofullerene was first observed in La Cs2 by the IBM Almaden group (Johnson et al., 1992) (Eigure 15) and was discussed within the framework of an intrafullerene electron transfer. The observation of eight equally spaced lines provides evidence of isotropic electron-nuclear hyperfine coupling (hfc) to La with a nuclear spin quantum number I = 7/2. The observed electron g-value of 2.0010, close to that measured for the Ceo radical anion (Allemand et al., 1991 Krusic et al., 1991), indicates that a single unpaired electron resides in the LUMO of the carbon cage. They also observed hyperfine... 

With the help of a selectively deuterated methylacetylene, the anisotropic hyperfine couplings are determined. Comparison of experimental values with the theoretical ones calculated by ab initio MO and INDO methods, a tran -bent structure is concluded. The formation of methylacetylene radical anion is also confirmed by an electron absorption spectroscopic study. 

The anion derived from 1,2-phenylenephosphorochloridite is one of the rare examples of a radiogenic radical anion produced by electron capture by a phosphine. In the case of arsenic compounds, AsCls" and AsFj" were produced by y-irradiation of polycrystalline trihalogenoarsines at 77 This last species exhibits hyperfine coupling with As (T// = 666MHz), two of one kind (r y = 240MHz) and one F of another kind (T// = 78MHz). These arsenic radical anions adopt an approximate planar T-shape structure with the unpaired electron located in an orbital perpendicular to the molecular plane. 

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