Zero-field splitting parameter

Fig. 9. Zero-field splitting of the S = 2 ground state of [Fe3S4] clusters. Constructed for D = -2.5 and E/D = 0.23, where D and E are the axial and rhombic zero-field splitting parameters, respectively.

The first term is characterized by a scalar, 7, and it is the dominant term. Be aware of a convention disagreement in the definition of this term instead of -27, some authors write -7, or 7, or 27, and a mistake in sign definition will turn the whole scheme of spin levels upside down (see below). The second and third term are induced by anisotropic spin-orbit coupling, and their weight is predicted to be of order Ag/ge and (Ag/ge)2, respectively (Moriya 1960), when Ag is the (anisotropic) deviation from the free electron -value. The D in the second term has nothing to do with the familiar axial zero-field splitting parameter D, but it is a vector parameter, and the x means take the cross product (or vector product) an alternative way of writing is the determinant form... 

In 1987 we reported26 that the three possible C3H2 isomers S-2, T-36, and S-37 can be transformed into each other under photochemical conditions. For several reasons propargylene (36) attracted our attention On the one hand the first C3H2 parent species, identified by direct spectroscopic methods, was triplet propargylene (T-36). Its ESR spectrum was published in 1965,63 and, based on the zero-field-splitting parameters, a linear or nearly linear structure was derived. On the other hand, the structural elucidation of 36 by comparison of the calculated and experimental IR spectra turned out to be rather difficult.64... 

Table 1.1. Zero-Field Splitting Parameters for Tyrosine and Tyrosinate0...

Table 4. Zero-field splitting parameters and derived bond angles for triplet carbenes...

The arylcarbenes are also bent, the angle being about 150—155° for phenyl-and diphenylcarbenes 63,64) jj g zero-field splitting parameters were shown to be appreciably dependent on the host matrix used >. However diphenylcarbene prepared from different precursors proved to be identical and is believed to be the triplet species of diphenylcarbene. 

Figure 10. EPR spectrum of [ZuFejS ] " recorded at 8 K. The lower panel shows energy levels and effective g>values for an S = 5/2 system with D < 0 and E/D = 0.25, where D and E are the zero field splitting parameters.

Table II. Zero-field Splitting parameters (in 10 cm ) and average triplet lifetimes of porphyrin monomers and dimers ...

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... 

Finally, one uses low-temperature techniques for ESR measurements of non-Kekule species not only because of the inherent high reactivity of the species, but also because unless the sample is immobilized or at least hampered in its motion by viscous solvent, rapid tumbling will cause the spectrum to suffer line shape distortions that impede the extraction of the zero-field splitting parameters. 

TABLE 9.14. Zero-Field Splitting Parameters" and Kinetic Parameters for Persistent Triplet Diarylcarbenes (106)... 

High-spin hemes have a strong axial zero field splitting which splits these states into three doublets, and because the value of the zero field splitting parameter D is positive (typically 5-10 cm ) for hemes the doublet consisting of the Sj = -1-, — states is lowest lying. There is usually also a smaller rhombic term E which cannot split the Kramers doublets but mixes states differing in by 2. The main net effect of E is to remove the equivalence of the x and y (in heme plane) directions so that g, and gy are unequal. 

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