X-band microwave quantum

For Cr(m) complexes, D is relatively small (comparable to the X-band microwave quantum, 0.317 cm-1) and all three fine structure lines are observable. This is not always the case. Consider high-spin Fe(m) in an axial ligand field with D > > hv0, E = 0. With the same Hamiltonian as above and the magnetic field along the z-axis, the energies are ... 

This corresponds to an EPR-silent sample that gives no detectable ESR spectrum at X-band frequencies because it possesses a zero-field splitting larger than the Zeeman interaction (see Chapter 6), and the energy spacing between the two lowest levels is too large to be spanned by a microwave quantum at X-band. Nevertheless, higher frequencies are able to induce transitions. Since... 

Here v is a (fixed) frequency of the microwave source (e.g. ca 9 GHz at X-band, 35 GHz at Q-band). The spectrum is taken as a first derivative of the absorption versus the ramping magnetic field Bz. According to the selection rules, only those transitions are observable (intense) for which the change in the quantum number of the projection of the spin angular momentum fulfils the condition... 

In all the spectra so far reported for high-spin Fe in proteins, the microwave quantum energy, hv, is much smaller at X-band frequencies ( 9-10 GHz) than the zero field splittings between the doublets (Griffith, 1964a Wickman et al, 1965 Oosterhuis, 1974). Thus ESR transitiofis only occur within each doublet and not between them. This circumstance leads to highly anisotropic g factors ( y = 2, = 6) when E = 0, and a... 

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