Hyperfine interactions

The hyperfine interaction provides the fine structure of the EPR spectrum. It is the interaction between the electron and nuclear magnetic moment, described by the Hamiltonian  

The S — Tq mixing mechanism can be shown more formally by considering a radical pair with only one magnetic nucleus (only the effect of the hyperfine interaction is considered). The energy levels of the hyperfine states can be calculated as 

The nuclear magnetic dipole moment can interact with a magnetic field, inducing a splitting of a nuclear state with spin quantum number I into 2/ + 1 equally spaced, nondegenerate substates. The energies of these substates are given by  

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Hyperfine Interaction (dipolar and scalar) 2,0 Electron relaxation, may be complicated Paramagnetic systems and Impurities [17-191... 

Closs G L and Trifunac A D 1970 Theory of chemically Induced nuclear spin polarization. III. Effect of Isotropic g shifts In the components of radical pairs with one hyperfine Interaction J. Am. Chem. Soc. 92 2183-4... 

The spectrum of radical 101 appears as a quintet (1 2 3 2 1) caused by the hyperfine interaction (HFI) with two equivalent nitroxide nitrogen nuclei (<2n = 0.74 mT), each line of the quintet being additionally split due to hyperfine... 

The value of the magnetic hyperfine interaction constant C = 22.00 kHz is supposed to be reliably measured in the molecular beam method [71]. Experimental data for 15N2 are shown in Fig. 1.24, which depicts the density-dependence of T2 = (27tAv1/2)-1 at several temperatures. The fact that the dependences T2(p) are linear until 200 amagat proves that binary estimation of the rotational relaxation rate is valid within these limits and that Eq. (1.124) may be used to estimate cross-section oj from... 

The origin of postulate (iii) lies in the electron-nuclear hyperfine interaction. If the energy separation between the T and S states of the radical pair is of the same order of magnitude as then the hyperfine interaction can represent a driving force for T-S mixing and this depends on the nuclear spin state. Only a relatively small preference for one spin-state compared with the other is necessary in the T-S mixing process in order to overcome the Boltzmann polarization (1 in 10 ). The effect is to make n.m.r. spectroscopy a much more sensitive technique in systems displaying CIDNP than in systems where only Boltzmann distributions of nuclear spin states obtain. More detailed consideration of postulate (iii) is deferred until Section II,D. 

Here and H describe radicals A and B of the radical pair and He the interaction of their electrons. The other terms in equation (15) are H g, the spin orbit coupling term, H g and Hgj, representing the interaction of the externally applied magnetic field with the electron spin and nuclear spin, respectively Hgg is the electron spin-spin interaction and Hgi the electron-nuclear hyperfine interaction. 

The summations are over all hyperfine interactions in A and B except that with nucleus i. ... 

OIDEP usually results from Tq-S mixing in radical pairs, although T i-S mixing has also been considered (Atkins et al., 1971, 1973). The time development of electron-spin state populations is a function of the electron Zeeman interaction, the electron-nuclear hyperfine interaction, the electron-electron exchange interaction, together with spin-rotational and orientation dependent terms (Pedersen and Freed, 1972). Electron spin lattice relaxation Ti = 10 to 10 sec) is normally slower than the polarizing process. 

Fe Q-band ENDOR study of the isotopically enriched Ni-C state of D. gigas and D. desulfuricans hydrogenases and Ni-B state of D. desulfuricans revealed a weak coupling between the Fe and the nickel atoms when the enzyme was in the Ni-A forms while no coupling was observed for the Ni-B form (186). A careful analysis of linewidth of Ni-A and Ni-B EPR signals detected in Fe enriched and nonenriched hydrogenase samples indicated that hyperfine interactions are lost in the spectral linewidth and, hence, nonde-tectable. 

Millet JM, Toyir J, Didillon B, Candy JP, Nedez C, Basset JM (1997) Hyperfine Interactions 108 477... 

Dengler J, Konig E, Larkworthy LF, Ritter G, Sengupta SK (1990) Hyperfine Interact 56 1443... 

Most Mossbauer spectra are split because of the hyperfine interaction of the absorber (or source) nuclei with their electron shell and chemical environment which lifts the degeneracy of the nuclear states. If the hyperfine interaction is static with respect to the nuclear lifetime, the Mossbauer spectrum is a superposition of separate lines (i), according to the number of possible transitions. Each line has its own effective thickness t i), which is a fraction of the total thickness, determined by the relative intensity W of the lines, such that t i) = Wit. 

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