Hyperfine splitting constants—

The spin adducts of free radicals and MNP or DMPO were observed by means of an ESR spectrometer. The data of hyperfine splitting constants were compiled in Tables 9 and 10 [40-42,44,45]. ESR studies on the initial free radicals revealed that the monoalkylamino radical RHN-, dialkylamino radical R2N-, and aminomethyl radical -CH2N< or aminoethylidene radical >N( CHCH3) were obtained from the corresponding primary, secondary, and cyclic tertiary amine. In case of a tertiary diamine such as TMEDA, formation of... 

Table 9 Hyperfine Splitting Constants of Spin Adduct Formed APS/Amine/MNP or DMPO System ...

Tabie 10 Hyperfine Splitting Constants of Spin Adducts Obtained from APS/Amine/MNP Systems... 

System Radical trapped by MNP Hyperfine splitting constant (0.1 mT) ap ap a," ... 

TABLE 3. Hyperfine splitting constants (gauss) of some sulfur-cpntaining aromatic radical anions obtained in 1,2-dimethoxyethane with K metal as the reductant at — 80 °C... 

TABLE 4. Hyperfine splitting constants (gauss) of thioxanthone S-(mono or dijoxide radical anions as free or tight contact ion-pair ... 

It can be noticed there are two types of hydrogen atoms atoms 1-5 lying in the molecular plane and atoms 6 and 7 lying out of the plane. Thus, the hyperfine splitting constants (an) cannot be interpreted by means of one relationship, but the expressions... 

Yonezawa and collaborators (99) have reported unrestricted open-shell SCF calculations where the one-center exchange integrals were taken into account their treatment concerned allyl, vinyl, and nitrogen dioxide radicals. The one-center exchange integrals also are involved in the INDO method (85). Here, the following relationship for hyperfine splitting constants holds ... 

Because of its distorted tetrahedral shape, and the long Cu-S(Met) bond, the type 1 Cu active site has proved difficult to model in low complexes. The unusual EPR spectra with characteristically small hyperfine splitting constants are a consequence of the asymmetry of the copper environment. 

The resonance expression relating the magnetic field and energy (frequency) as seen in equation 3.40, A =/iv=gP5ij, has its analog for hyperfine interactions written as equation 3.43, where A is the hyperfine coupling constant in Hz and a is the hyperfine splitting constant—that is, the distance between the split lines in the EPR spectrum ... 

The effect of the sulfone dipole on the ESR spectrum of the dibenzothiophene 5,5-dioxide anion has been examined, modified Hiickel calculations which ignored d-orbital conjugation predicting spin distributions which agreed with experimental data. Proton hyperfine splitting constants have also been obtained for the sulfone. Correlation with MO calculations shows that the sulfone group contributes a vacant symmetric orbital to the conjugated system. ... 

As mentioned before, chromium(VI) reacts with NADH or NAD(P)H in the presence of rat-liver cytosolic or microsomal fractions, forming stable Cr(V)-NAD (P)H complexes, while addition of Glc6P led to stable Cr(V)-Glc6P complexes together with the Cr(V)-NAD(P)H complexes.11 The chromium(V) complexes have isotropic g values of 1.980 or 1.982, and H hyperfine splitting constants of... 

As Stated in the previous section, reduction of thieno[3,2-2 ]thiophene (2) with Na-K alloy at —100 results in the formation of a radical-anion. With AlClj in nitromethane at —20°, or SbClj in methylene chloride at —60°, a radical-cation was obtained. The experimental hyperfine splitting constants (HFSC) data are shown in Eq. (73). 

The electron spin resonance (ESR) spectra of the radical anion of 2,2 -bipyridine, sometimes in the form of its alkali metal com-plgx, 71.175,177.299-304 radical anion of 3,3 -bipyridine, ° and the radical anion of 4,4 -bipyridine, ° ° usually obtained by reduction of the bipyridines with an alkali metal, have been measured, and hyperfine splitting constants were assigned. Related biradical species have also been investigated. The ESR spectrum of the 4,4 -bipyridinium radical cation, of which... 

Here / is a nuelear spin, F is the full momentum of the system, and J is the full electron momentum. The hyperfine splitting constants are expressed through the standard radial integrals ... 

As mentioned for the relationship between the PE spectrum of a parent molecule and the electronic spectrum of its radical cation, any close correspondence between the electronic spectra of anions and cations or their hyperfine coupling patterns holds only for alternant hydrocarbons. The anions and cations of nonalternant hydrocarbons (e.g., azulene) have significantly different hyperfine patterns. Azulene radical anion has major hyperfine splitting constants (hfcs) on carbons 6, and 4,8 (flH = 0-91 mT, H-6 ah = 0-65 mT, H-4,8 ah = 0-38 mT, H-2) in contrast, the radical cation has major hfcs on carbons 1 and 3 (ah = 1.065 mT, H-1,3 Ah = 0.152 mT, H-2 ah = 0.415 mT, H-5,7 ah = 0.112 mT, H-6). °°... 

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. 

Figure 3.4 (a) Catalytic efficiencies of subtilisin Carlsberg in THF with different water contents, (b) Polarity of subtilisin s active site (as indicated by the hyperfine splitting constant, A0, of the active-site bound spin label) with different water contents [54]. 

If polyethylene is irradiated at higher temperatures—e.g., at room temperature—a new spectrum containing seven lines, with the hyperfine splitting constant of 21 gauss, is obtained (39). This spectrum is attributed to the allylic radicals (II) ... 

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