Zero-field splitting electron spin 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... 

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... 

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... 

On the other hand, the cyanomethylene H—C—C=N triplet as well as a number of odd alternant methylene compounds, propargylene and its homologues, H— —C=CR (R = H, CH3, C6H5), and H—C— C=CC=CR [R = CH3, C(CH3)3, C6Hs] have been predicted to be linear on the basis of the zero-field splitting parameters from electron spin resonance (ESR) experiments (15). The linear nature of the first named H—C—C=N triplet carbene has been confirmed by a micro-wave study in the gas phase. However, if the potential well in which the linear configuration lies is shallow, the ground state may not show the effects of nonlinearity (142). 

ENDOR = electron nuclear double resonance EPR = electron-paramagnetic resonance ESR = electron-spin resonance NMR = nuclear magnetic resonance MA = modulation amplitude SOFT = second-order perturbation theory s-o = spin-orbit zfs = zero-field splitting (for S > 1 /2) D = uniaxial zfs E = rhombic zfs g = g-factor with principal components gy, and g ge = free electron g-factor a = hyperfrne splitting constant A = hyperftne coupling constant for a given nucleus N (nuclear spin 7>0). 

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