ESR Spectra Of Radical Anions Research Articles

Effects of Lowering Symmetry on the ESR Spectra of Radical Anions of Fullerene Derivatives and the Reduction Potentials

Radical anions of dibenzyl adducts of C60, 1,4-(C6H5CH2)2C60•- and 1,2-(C6H5CH2)2C60•-, give ESR spectra which have larger g values (2.0004 and 2.0001, respectively) and much smaller line widths (ΔHmsl = 2.5 and 3.3 G, respectively, at 213 K) than the g value (1.9984) and the ΔHmsl value (30.9 G at 213 K) of C60•-. An even smaller ΔHmsl value (0.17 G) and a larger g value (2.0011) are observed for the tetrabenzyl C60 adduct radical anion, 1,4,10,24-(C6H5CH2)4C60•- and this is ascribed to a large splitting of the degenerate t1u orbitals caused by introduction of four benzyl groups to C60. In this case, a hyperfine structure, due to two nonequivalent protons of only one benzyl group (aH1 = 0.31 G, aH2 = 0.11 G), is observed and this is consistent with the predicted localized spin density at the C2 position next to the C1 carbon to which a benzyl group is attached. The radical anions of the mono- and bisadducts formed in the Diels−Alder cycloaddition reaction of C60 with 9,10-dimethylanthracene show ESR signals at different g values (2.0003 for the monoadduct and 2.0009 for the bisadduct). The relationship between lower symmetry and the ESR spectra of radical anions of various C60 derivatives is discussed in terms of the g values and the line widths. The energy gap (δ) between the singly occupied orbital and the two other orbitals which had a t1u symmetry prior to introduction of addends to C60 is derived from differences in the g values of various C60•- derivatives from the free spin value (2.0023). A linear correlation is shown to exist between log ΔHmsl and −δ. The δ values are also obtained from Arrhenius plots of ln ΔHmsl vs T-1 and they agree well with the values derived from the g values and the reduction potentials.

ESR study of radicals formed by the reduction of α-diketones C(O)C(O)CF3 (R=(CF3)2CF, C6F5, (CF3)3C) with several group I–III metals

The ESR spectra of radical anions formed by reduction of α-diketones RC(O)C(O)CF3 (R=(CF3)2CF, C6F5, (CF3)3C) with metals (Li, Na, K, Mg, Cd, Zn, Hg, In, and TI) in THF were studied. For R=(CF3)2CF and C6F5, the radical anions are formed ascis-isomers, whereas for R=(CF3)3C,trans-isomers are obtained. Line broadening due to solvation and desolvation of the cation is observed in the latter case. The reduction of α-diketone (CF3)2CFC(O)C(O)CF3 with Group II metals (Mg, Cd, Zn) results in the formation of radical pairs.

ESR spectra of radical anions ? Products of the reaction of fullerene with alkali metals

The ESR spectra of the products of the reaction of fullerene C60 with lithium, sodium, and potassium, including doping in time, were described. The formation of a radical anion with homogeneous reduction of fullerene by samarium iodide in solution was demonstrated.

The effects of site exchange on the ESR spectra of free radicals with radical pair mechanism chemically induced dynamic electron polarization (CIDEP)

The possible effects of exchange processes upon polarized ESR spectra are discussed using the amended Bloch equations. The results are applied to an analysis of the polarized ESR spectrum of the cyclohex-1-olyl radical, observed over a range of temperatures using the time integration spectroscopy technique. The spectrum obtained at 223 K displays an unusual E/A/E/A polarization pattern, the full analysis of which is given. In the light of these results, the fact that the polarized ESR spectra of radical anions usually fail to show any radical pair mechanism polarization is discussed.

On the mechanism of regioselectivity control by donor substituents on electrophilic alkenes: applications to quinone cycloadditionregioselectivity

Donor substitution on alkenes slightly increases the LUMO coefficient at the site of substitution. However, nucleophiles preferentially attack the unsubstituted terminus of the alkene due to direct repulsive interactions between the nucleophile and the substituent. Secondary orbital interactions make the terminus remote from the site of substitution the most electrophilic. LUMO coefficients, or equivalent spectroscopic measures, esr hyperfine coupling in the esr spectra of radical anions, are appropriate for prediction of reactivities of unsubstituted positions but not substituted ones.

Radical-anion ion pairs with a six-coordinated silicon atom and nonequivalent ligands

1. A study of the ESR spectra of radical anions containing a six-coordinated Si atom and nonequivalent ligands has shown that interligand bond and unpaired-electron exchange depends on the degree of cation solvation and the nature of the ligand. 2. The data suggest that bond and unpaired-electron exchange also occurs in the corresponding biradicals.

Quantum-chemical study of radical ions and molecules incorporating solvent effect.: II. Calculation of electronic transitions and spin densities of various conjugated radical anions in π-approximation

Possibilities of improving the interpretation of electronic and ESR spectra of radical anions derived from aza, cyano, keto and nitro derivatives of aromatic hydrocarbons within the restricted SCF π method are investigated. With regard to the standard parametrization (method A) two modifications are made: (i) incorporation of the polarization of the σ skeleton and the effect of the non-uniform distribution of π-electrons (method B), (ii) incorporation of the solvent effect directly into the Fock operator according to the procedure proposed by Germer (method C). It is shown that method B overestimates the σ-polarization and yields poorer results as compared to the standard method A. On the contrary, inclusion of the solvent effect directly in the Fock operator (method C) is shown to be significant for the mentioned systems of radical anions and convenient for characterization of this effect — improvement of the electronic and ESR spectra being considerable as compared to methods A and B.

ESR Spectra of Radical Anions of Some 6a‐Thiathiophthene Derivatives

ESR Spectra of Radical Anions of Some 6a‐Thiathiophthene Derivatives

ESR Spectra of Radical Anions of Aliphatic Azo Compounds: tert‐Butyl and Trimethylsilyl Derivatives of Diimine

ESR Spectra of Radical Anions of Aliphatic Azo Compounds: <i>tert</i>‐Butyl and Trimethylsilyl Derivatives of Diimine