Electron hyperfine lines

Clearly, the eight hyperfine lines (7 = 7/2 for 51V) have different widths but careful examination also shows that the line spacing varies, increasing with increasing B. To understand the origin of this effect we must take a closer look at the solutions to Eqn. (3.1) for the case of an unpaired electron interacting with a single nucleus. This will lead us to a derivation of eqns (2.5) and (2.11) of Chapter 2. 

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... 

The hyperfine interaction is shown in Figure 21 of reference 16. The Ms = 1/2 states of an S = 1/2 paramagnet interact with an I = 1/2 nuclear moment to create the hyperfine interaction. Interactions from ms = — 1/2 to Mi = - 1/2 and ms = - 1/2 to Mi = + 1/2, for instance, create the magnetic field specified as the hyperfine interaction A. Figure 21 of reference 16 describes the behavior for an 1=1/2 nuclear moment. The number of hyperfine lines will be equal to 21 + 1 for nuclear moments greater than 1/2. Each hyperfine line will be of equal intensity when the electron is interacting with its own nucleus. For instance the Cu2+, / = 3/ 2 nucleus will produce four hyperfine lines as described in the next section. 

Oxovanadium(IV) complexes with dithiophosphate ligands have been extensively examined <8,121.161,252,386) x typical ESR spectrum is shown in Fig. 7. In addition to the eight vanadium 1=112 hyperfine lines phosphorus (/ = 1/2) superhyperfine splitting is also observed. The phosphorus superhyper-fine splitting can be considered a bit unusual since the phosphorus is located about 3 A or more away from the metal ion. P and As superhyperfine splitting has been observed in the ESR spectra of ill-defined vanadium phosphine 388) and arsine 389) complexes but in those cases, presumably, direct V-P and V—As interactions occur. ESR parameters have been tabulated for a large number of dithiophosphate 121,252) dithiophosphinate 121.252) complexes. Evaluation 3i) of the fractional 3s character of unpaired electron in dithiophosphate complexes yielded a value of 1.35%. The vanadyl(IV) complexes possess approximate C2V symmetry. The unpaired d electron resides... 

The transitions in the X-band ESR spectra of triplet species occur in two regions. The so-called Anis = 1 region represents transitions between energetically adjacent pairs of the three triplet sublevels. These are characterized by two so-called zero-field splitting parameters, D and E. The parameter D is inversely proportional to the cube of the average separation of the electron spins, and E is related to the molecular symmetry. The number of lines depends on the molecular symmetry. If all three magnetic axes of the molecular carrier of the spectrum are distinct, the spectrum in the Anis = 1 region will show six major resonances, plus any hyperfine lines that may be visible. If two of the principle axes are equivalent by symmetry, only four lines will be observed. In the latter case, the parameter E has the value of... 

Frozen-solution ESR spectra of Tc2G in mixed aqueous hydrochloric acid and ethanol provided data consistent with equal coupling of the unpaired electron to both technetium nuclei (101). IsotopicaUy pure "Tc (/ = 9/2) in 99Tc2Cl leads to a large number of lines in the X-band spectrum owing to second-order effects, in addition to the hyperfine lines presence for this dimeric axially symmetric system. The Q-band spectrum obtained at 77°K with a microwave frequency of 35.56 GHz exhibited fewer lines, and computer-simulated spectra were generated to correspond to the experimental spectrum withgit = 1.912, gi = 2.096, An = 166 x 10 4 cm"1, IAL = 67.2 x 10 4 cm 1, and gav = 2.035. 

Amide. — It has been pointed out before that europium behaves more or less like the alkaline earths and is closely related to strontium and barium. It is found to react with liquid ammonia at —78° C in much the same way as the alkali metals forming a characteristic deep blue solution. Eu(NH2)2 can be isolated [260] from the blue solution. Recent electron paramagnetic studies [261] of solutions of europium in liquid ammonia showed the presence of complex hyperfine lines arising from Eu2+ (8 7/2, g — 1.990 0.002) besides the characteristic single line of the solvated electron (g = 2.0014 0.0002) K The departure of the Eu2+ <7-value from the free electron value is explained as being due to spin-orbit coupling and a slight admixture (3.5%) of the 6P7/2 state. 

The separation (A) of the hyperfine lines in the ESR spectra of metal-amine, and metal-ether solutions represents a direct measure of the average s-electron (spin) density of the unpaired electron at the particular metal nucleus (12,156). When this splitting is compared to that of the free (gas-phase) atom, we obtain a measure of the "percent atomic character of the paramagnetic species. The percent atomic character in all these fluid systems increases markedly with temperature, and under certain circumstances the paramagnetic species almost takes on "atomic characteristics (43, 53, 160). Figure 9 shows the experimental data for fluid solutions of K, Rb, and Cs in various amines and ethers, and also for frozen solutions (solid data points) of these metals in HMPA (17). The fluid solution spectra have coupling con-... 

Three membrane-bound adenosine triphosphatase enzymes have been characterized using Mn(II) and Gd(III) electron paramagnetic resonance (EPR) and a variety of NMR techniques. Mn(II) EPR studies of both native and partially delipidated (Na+ + K+)-ATPase from sheep kidney indicate that the enzyme binds Mn2+ at a single, catalytic site with Kq = 0.21 x 10- M. The X-band EPR spectrum of the binary Mn(II)-ATPase complex exhibits a powder line shape consisting of a broad transition with partial resolution of the 55 n nuclear hyperfine structure, as well as a broad component to the low field side of the spectrum. ATP, ADP, AMP-PNP and Pj all broaden the spectrum, whereas AMP induces a substantial narrowing of the hyperfine lines of the spectrum. 

From here, the saturation factor requires further discussion. Equation (9) is correct for radicals with a single ESR transition however, the picture becomes more complicated for radicals with more than one transition due to hyperfine splitting. The nitroxide radicals commonly used for ESR and DNP fall into this category,47 48 as the impaired electron in these molecules partially resides on a nitrogen nucleus with spin 1 (14N) or spin 1/2 (15N) giving three or two hyperfine lines, respectively. For the more common 14N nitroxide radicals, at low concentrations in aqueous solutions the right side of Equation (9) is multiplied by a factor of 1 /3, as only one hyperfine line can be saturated at a time.49 However, two processes can serve to mix the hyperfine lines and increase the saturation factor in the limit of infinite power (smax) of nitroxide radicals well beyond smax = 1/3. 

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