Large hyperfine couplings

The line positions for each /eg are not centred at the field Bq corresponding to the g-factor, but are displaced as shown in the stickplot of Fig. 3.5 for a radical with a large hyperfine coupling of six equivalent I = V2 nuclei (data are for C3F6 in Chapter 5) with effective nuclear spin values, /eff = 0, 1, 2, and 3 which occur 5, 9, 5 and 1 times. The 2 leg +1 hyperfine lines for a specific value of Lg V2 are not equally spaced but the line splittings increase towards higher fields. The 

The stickplot can be obtained by manual calculation, but is also conveniently computed with a numeric procedure to obtain the effective spin values leg and their multiplicities Ni in a computer algorithm described in Section 3.2.7. The procedure to obtain the -factor when 2nd order effects occur is described in Section 3.2.7, see also Exercise E3.4. Examples of spectra showing second order effects are given in Chapter 5 for perfluoro-cycloalkane radical anions, with several equivalent nuclei with large hyperfine couplings. 

Not all ESR laboratories can afford the price and running costs of a high field ESR spectrometer equipped with a superconducting magnet. Applications like those considered in Chapter 5 are also most conveniently performed at X-band. Thus the need for analysis beyond first order remains. 

A troublesome complication is caused by the overlap of spectra due to different paramagnetic species considered in the next section. Spectrum simulations described in Section 3.2.7 are often applied in this case to refine the hyperfine coupling constants. 

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The relatively large hyperfine coupling to ring protons was unexpected, but was quickly interpreted by various workers (Bersohn, 1956 McConnell, 1956 Weissman, 1956) in terms of configurational interactions which give rise to spin polarization of the cr-C—H bonding... 

The rather large hyperfine coupling of about 1.7 mT [49b] from two... 

The lack of large hyperfine couplings (18-27 G) in the ENDOR spectra of coals is interesting from another point of view. It precludes the existence of radicals having their unpaired spins strongly localized on... 

Amino-boranes and other Compounds containing B—N Bonds.— Trimethyl-amine-borane, on exposure to "Co y-rays at 77 K, forms the radical-anion McjN—BH3. This has a four-line e.s.r. spectrum, with a large hyperfine coupling to the boron atom. Annealing this species at temperatures >77 K gives Mc2N— BH,. ... 

S-To-type, and reports of S-T i-type CIDNP imder these conditions are very rare, even in low fields. All recent examples involve radical pairs with very large hyperfine coupling constants caused by an inorganic nucleus ( P or Sn). Even then, diffusion must favour longer dwell times around the level crossing, as is seen by the observation that low-field net CIDNP of P from the radical-pair trimethylbenzoyl/dimethoxyphosphonyl still arises to 60-70% through S-To-type CIDNP in the less viscous solvent acetonitrile, while in the more viscous solvent dioxane the contribution of this intersystem crossing pathway decreases to 20-50%. ... 

MHz) so that spectra at multiple magnetic fields can be easily compared to show hyperfine coupling. The latter situation, a Larmor-split hyperfine-centered doublet, is typically found for Fe, which has a very small g value but large hyperfine coupling since the nucleus is usually an intrinsic part of the paramagnetic center. Such a situation at X-band would yield, for example, v 14.5 MHz, 15.5 MHz at X-band (where 0.5MHz, A 30MHz). The abscissa of... 

Consider firstly the T initial case. Since we assume that only the singlet state is reactive, recombination can only occur from a T pair if ST mixing occurs before the radicals finally diffuse apart. Except for radicals with very large hyperfine coupling, the ST interconversion is rather slow compared to the rate of diffusion in non-viscous solution so that the reaction probability is generally small. This permits us to make the approximation that in equation (10)... 

The visual analysis of the specfrum in Fig. 3.19 is less accurate than that in Fig. 3.18 due to the lower resolution, but proceeds similarly. The procedure is shown in the figure. The large hyperfine coupling makes it necessary to include second... 

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