33S hyperfine couplings

Spin density reported as a range by Maki and co-workers (387 and 388) determined from the slNi hyperfine and spin density for S depends on the sign the 33S hyperfine coupling constant (0.13 vs. 0.20) as discussed in (381). 

TABLE 33. Hyperfine coupling constants (G) of phenoxyl radical and phenol radical cation"... 

RADIATION CHEMISTRY H Table VI. Magnetic Parameters 14N, 15N, 31P, or 33S Hyperfine Coupling constants (gauss) ... 

The complex ion (Figure 2.32) contains Rh2 bound cis to two phosphorus atoms (2.216 A) and more distantly to four oxygens (2.201—2.398 A), exhibiting a distortion ascribed to the Jahn-Teller effect it is paramagnetic (fi = 1.80 fiB) and exhibits an ESR spectrum (Figure 2.33) showing rhodium hyperfine coupling as the doublet for g. ... 

The thymine anion is only a weak base = 6.9) [22]. This means that protonation of the anion may depend on the specific environment. The primary reduction product observed in the solid state in thymine derivatives is the C4-OH protonated anion [17]. This species exhibits significant spin density at C6 and 04. Here one must distinguish between two different situations. In single crystals of thymidine, the C4-OHp proton is out of the molecular plane which gives rise to an additional 33.1-MHz isotropic hyperfine coupling [31]. A similar situation is observed in single crystals of anhydrous thymine [32]. In 1-meThy, however, the C4—OHp proton is in the molecular plane. Consequently, the proton coupling is very small. 

The adenine radical cation was observed in a single crystal of adenine hydrochloride hemihydrate [43]. In this crystal, the adenine is protonated at Nl. After electron loss, the molecule deprotonates at Nl, giving Ade(Nl -l-H, Nl-H). This produces a radical that is structurally equivalent to the cation of the neutral adenine molecule with spin density on C8 and N6 [p(C8) = 0.17 and p(N6) = 0.25]. The adenine radical cation is strongly acidic (pi a< 1) [22]. This strong driving force makes the reaction independent of environmental conditions. In single crystals of adenosine [42] and anhydrous deoxyadenosine [44], the N6 deprotonated cation [Ade(N6-H) ] is observed which is characterized by p(C8) = 0.16 and p(N6) = 0.42. The experimental isotropic hyperfine couplings are N6-H = 33.9 MHz and C8-H = 12.4 MHz. 

Table 33. Selecteda) examples of correlation between experimental and calculated isotropic hyperfine coupling constants for 13C...

For some of these compounds, the protonic hyperfine coupling constants are known. The isotopic A /Ap ratio is 1.21 for radicals produced by addition to C6H6 and to the ortho position of C6H5CH3 and 1.15-1.18 for several fluorobenzenes (133). Quantum chemical calculations that include averaging over 33 vibrational modes in CeH7-C6H6Mu have shown that the dynamics account quantitatively (A /Ap = 1.16) for the... 

During the past two decades, five crad scales have been proposed to assess electronic effects in para- and meta-substituted benzyl radicals, four of which are of the kinetic type based on chemical reactivity. For example, the Fisher scale [Eq. (11)] [32] involves the A -bromosuccinimide(NBS)-initiated hydrogen abstraction from aryl-substituted m-cyanotoluenes. For the Jackson scale [Eq. (12)] [33], the thermolysis of dibenzylmercury compounds was employed. The most comprehensive scale is that by Creary [34] [Eq. (13)], which was derived from the relative rearrangement rates of 2-aryl-3,3-dimethylmethylenecyclopropanes. The most recent kinetic scale comes from Jiang et al. [35] [Eq. (14)], who applied the dimerization rates of substituted trifluorostyrenes for this purpose. Only Arnold s ax scale [36] is of the spectroscopic type, based on EPR a hyperfine coupling constants of photo-lytically generated aryl-substituted benzyl radicals [Eq. (15)]. 

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