Mixed hyperfine interaction

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. 

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. 

Hyperfine interaction has also been used to study adsorption sites on several catalysts. One paramagnetic probe is the same superoxide ion formed from oxygen-16, which has no nuclear magnetic moment. Examination of the spectrum shown in Fig. 5 shows that the adsorbed molecule ion reacts rather strongly with one aluminum atom in a decationated zeolite (S3). The spectrum can be resolved into three sets of six hyperfine lines. Each set of lines represents the hyperfine interaction with WA1 (I = f) along one of the three principal axes. The fairly uniform splitting in the three directions indicates that the impaired electron is mixing with an... 

Striking evidence for the role of the hyperfine interaction on the time dependence of ion-pair recombination was found by Brocklehurst [56, 409]. He ionized a solution of p-terphenyl in decalin (benzene, squalane etc.) with 90Sr (3 particles and observed the time dependence of the fluorescence intensity. When the experiment was repeated, but with a magnetic field (< 0.16 T) applied, the fluorescence intensity was enhanced by approximately 40% after about 10—20 ns. The magnetic field splits the triplet levels so that only T0 can mix with S0 and the initial singlet... 

Figures 49 and 50 show the 600-MHz H NMR spectra of cross-linked mixed-valency asymmetric hybrid Hbs with one cyanomet chain per Hb tetramer. Figures 49A and 50A give the proton resonances of (a+CNP)A(aP)cXL and (aP+CN)A(a(3)cXL in D20 from +7 to +20 ppm from HDO. These hyperfine-shifted resonances arise from the protons in the heme groups and/or the protons of the amino acid residues in the immediate surroundings of the heme group. They are shifted from their usual diamagnetic positions by hyperfine interactions between the unpaired electrons from the low-spin ferric and high-spin ferrous heme iron atoms and the nearby protons. As discussed earlier, they are very sensitive to the environment of the heme pocket. The signals from the subunits with high-spin ferrous heme iron are much broader than those from cyanomet (low-spin...

Energy level diagram and S-T transitions in a radical pair at a high external magnetic field (a) without and (b) with the resonance microwave field hfi is S-To mixing due to hyperfine interactions ESR is resonance microwave transitions. (Reproduced from Ref. [13b] by permission from The American Chemical Society)... 

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