Large zero-field splitting

Sweeney, W. V., D. Coucouvanis et al. (1973). ESR of spin 5/2 systems with axial symmetry and moderately large zero-field splittings. Application of line-shape calculations to the interpretation of randomly oriented microcrystallite spectra. J. Chem. Phys. 59 369-379. 

The diaqua and aqua (hydroxo) hemin complexes encapsulated in the micelles [20] are found to be high-spin (peff = 5.7 — S.Sps). Their high-spin nature is further confirmed from the ESR spectra at 4.2 K (Fig. 4). The spectra are characteristic of high-spin ferric porphyrins with a large zero-field-split Ai ground state with Mg = 1/2 lying lowest. The spectra are axially symmetric (gf = 2.05, = 6.0) for the diaqua complex, while for the aqua (hydroxo)... 

Values of the spin Hamiltonian reported for d4 ions are given in Table XI. The difficulty in detecting the ESR is due most likely to short spin-lattice-relaxation times and large zero-field splittings. In both octahedral and tetrahedral fields the 5 D state of d4 gives degenerate orbital states which,... 

It is important to realize that no one method of spectroscopy is clairvoyant. Electron paramagnetic resonance spectroscopy cannot sense low-spin Fe(II) as this state is diamagnetic, nor reliably the high-spin Fe(II) state because of rapid spin-lattice relaxation, large zero-field splittings or both Mossbauer spectroscopy cannot distinguish Fe(III) spin states... 

Not all encounters in the reaction of 02 with eaq" lead to 02 ", and the appropriate spin factor for this is 2/3 because of the large zero-field splitting of triplet 02 (Schmidt et al. 1995). Similarly, spin dephasing is observed for the reaction of H with 02 (Han and Bartels 1994), and this may apply also to other R plus 02 reactions. 

Under certain conditions, particularly when the enzyme is incubated with Gd3+ at low temperature (2-4°C), the spectrum of the ATPase-Gd " " complex exhibits several low-field transitions, as well as a shoulder to the low-field side of the main transition (Figure 14A). This spectrum is similar to previously reported glassy spectra of transition metal complexes with large zero-field splittings in that they are broad and not centered at g = 2 (56,57,58,59). These spectra are unique in that all have a number of well-defined features at low field (see also ref,... 

Another reason for undetectability, if S > 1, is a large zero-field splitting parameter (D) relative to X-band quanta this usually... 

The motivation for this intense activity is the resolution enhancement available from higher Larmor fields, which enables small g-tensor splittings to be readily observed (Earle et al., 1994). For systems with large zero field splittings (Lynch et al., 1993), high-field spectra can be much simpler to analyze than X-band spectra, which increases the reliability and eases the interpretation of the data. In the study of fluid media (Earle et al., 1993), the increased importance of the g-tensor contributions vis-a-vis the hyperfine tensor contributions (e.g., for nitroxide spin labels) gives information that is complementary to lower-field studies. These concepts are discussed more fully elsewhere (Lebedev, 1990 Budil et al., 1989), and we refer the reader to the references for a more complete discussion. 

The spectra of the low lying states deduced from the magnetic data are shown in Fig. 10. Both [CuFe] and [CuCr] are EPR silent owing to the large zero-field splitting of the S = 2 state. 

The absolute positions of the doublet states are overestimated by about 2500 cm-1. The spin-orbit coupled emitting doublet states is predicted to be the 2A(3/2) component of the 2Eg state in agreement with spectral assignment but in conflict with excited state EPR experiments. Despite the quality of the calculations the large zero-field splitting (ZFS) of the 2Eg (220-290 cm-1) has not been reproduced with theoretical values less than 100 cm-1. 

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