Zero-field splitting resonance

The spectrum of Mn2+ in zeolites has been used to study the bonding and cation sites in these crystalline materials. This is a 3d5 ion hence, one would expect a zero-field splitting effect. A detailed analysis of this system was carried out by Nicula et al. (170). When the symmetry of the environment is less than cubic, the resonance field for transitions other than those between the + and — electron spin states varies rapidly with orientation, and that portion of the spectrum is spread over several hundred gauss. The energies of the levels are given by the equation... 

De Groot, M.S. and Van der Waals, J.H. 1960. Paramagnetic resonance in phosphorescent aromatic hydrocarbons. II Determination of zero-field splitting from solution spectra. Molecular Physics 3 190-200. 

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... 

This mechanism leads to a highly spin-polarized triplet state with a characteristic intensity pattern in the EPR spectrum, which is observed by time-resolved techniques (either transient or pulse EPR). The zero field splitting (ZFS) of the triplet state, which dominates the EPR spectrum, is an important additional spectroscopic probe. It can also be determined by optical detection of magnetic resonance (ODMR), for a review of the techniques involved and applications see reference 15. These methods also yield information about dynamical aspects related to the formation, selective population and decay of the triplet states. The application of EPR and related techniques to triplet states in photosynthesis have been reviewed by several authors in the past15 22-100 102. The field was also thoroughly reviewed by Mobius103 and Weber45 in this series. 

In a few papers an attempt has been made to interpret quantum-mechanically the zero-field splitting parameters, which have been determined from the analysis of the electron spin resonance spectra on the triplet state of the pyrimidine bases. 

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... 

Zero-field splitting adds complexity to the spectra. For the case of an axial symmetry distortion, the resonance fields for various Ms —> Ms + 1 transitions can be derived from a perturbation approach 14... 

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