Methyl radical hyperfine coupling constant

When ESR spectra were obtained for the benzene anion radical, [C6II6] and the methyl radical, CH3, the proton hyperfine coupling constants were found to be 3.75 and 23.0 G, respectively, i.e. they differ by about a factor of 6. Since the carbon atom of CH3 has a spin density corresponding to one unpaired electron and the benzene anion carries an electron spin density of 1/6, the two results suggest that the proton coupling to an electron in a n-orbital is proportional to the spin density on the adjacent carbon atom ... 

The MesSn" radical appears to couple to only two of the methyl groups. Moreover, the isotropic and anisotropic tin hyperfine coupling constants indicate that the Sn 5s and 5p orbital contributions are roughly 0.03 and 0.32, respectively (Table 6). Thus, compared... 

Arnold s scale is derived for the action of a single substituent on the benzylic 7c-system. It cannot be used to estimate the influence of several substituents on the system under consideration. In this way it is, therefore, not possible to gain insight into the problem of captodative stabilization of a radical centre. The investigation of the spin-density distribution in benzylic radicals has been extended (Korth et al., 1987) to include multiple substitution patterns. Three types of benzylic radicals were considered a,p-disubsti-tuted a-methylbenzyl radicals [17], a-substituted p-methylbenzyl radicals [18] and a-substituted benzyl radicals [19]. In [17] and [18] the hyperfine coupling constants of the methyl hydrogens were used to determine the spin-density... 

Oxidation of a snlfide to sulfoxide is known to be an electrophilic reaction, in contrast with nncleophUic oxidation of sulfoxide to sulfone. Since 2-nitrobenzenesulfinyl chloride/KOi oxidizes sulfides to sulfoxides selectively, intermediate 48 must be the actual active intermediate. Moreover, in the presence of l,4-diazabicyclo[2.2.2.]octane (DABCO), which is a radical capturing reagent, the oxidation of methyl phenyl sulfide to the sulfoxide was inhibited. In order to further detect the intermediate 48, pure 5,5-dimethyl-l-pyrroUne-l-oxide (DMPO) was used as a trapping reagent and spin adduct was obtained. The ESR spectrum of the DMPO spin adduct was obtained by the reaction of 02 with 2-nitrobenzenesulfinyl chloride (hyperfine coupling constants, aH = 10.0 G and aN = 12.8 G). 

Table 37. Calculated and observed isotropic hyperfine coupling constants for methyl radical and isotopically substituted derivatives...

The methyl radical CHs has also been extensively studied, particularly with respect to the hyperfine coupling constants, to which we refer elsewhere.176 However, more recent work with various large GTO basis sets has appeared, with energy minimiza-... 

A typical example is seen for 1-hydroxyethyl radical trapped in a y-irra-diated ethanol matrix at 77 K [16]. As is shown in Fig. 2, the cw ESR spectrum of the radical is composed of five lines due to hyperfine interactions with one a proton and three P protons of a methyl group. The hyperfine interaction depends on the location of the P protons with respect to the p. orbital of the unpaired electron. However, the observed hyperfine coupling constant is the same for all the p protons because of the quick rotation of the methyl group in the time scale of the cw ESR measurement. On the other hand, the ESE-detected ESR spectrum is composed of four lines due to the hyperfine interactions with... 

The hyperfine splittings in the e.s.r. spectra of radicals of the allylic type are considerably less than those of alkyl radicals, and for radicals trapped in their parent compounds the resolution is insufficient to determine all the hyperfine coupling constants. However, by use of the rotating cryostat, the unsubstituted radical and three methyl-substituted allyl radicals have been prepared in a matrix of adamantane and it has been possible to resolve all the hyperfine couplings. 

The fast motion spectrum of the /-PMMA radical consists of 21 lines attributed to three separate isotropic hyperfine coupling constants. There is coupling to the methyl group to form a quartet (22.9 G) that is then split further into a triplet from one set of p-methylene protons (16.4 G) and another triplet from the other set (11.7G). Theoretically, this should lead to 36 lines (4 x 3 x 3), but a fortuitous degeneracy exists because one of the fast motion p-methylene couphng constants is almost exactly... 

The calculated hyperfine coupling constants (B3LYP/6-31G //MP2/6-31G ) for the type B transition state and the distorted minimum clearly show that this species must be considered a type A structure. The hyperfine coupling pattern of the lowest-energy minimum [ai = —1.43 mT a = 1.98 mT) shows a trend similar to the experimental splittings of the fran.s-l,2-dimethylcyclopropane radical cation a, 2 = —1.19 mT a = 2.18 mT), whereas the pattern calculated for the transition state (fl2,3 = 0.55 mT) is incompatible with that model (Figure 18). The distorted structure type calculated for the methyl-substituted systems seems to prevail also under other conditions (see below). 

Free radicals are generally short-lived, highly reactive species, usually characterized experimentally by their magnetic properties only. Thus a successful theoretical approach must be able to provide at the same time reliable structural and magnetic properties. Here we have chosen as representative models the methyl, aUyl and formaldehyde cation radicals. The isotropic hyperfine coupling constant (hcc) of a magnetically active nucleus N (a(N)) is related to the spin densities at the nucleus by [69]... 

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