Overcoming large zero-field splitting
The effect of a spin Hamiltonian including both a Zeeman, and second-rank zero-field splitting (zfs) acting on a triplet (S = 1) wavefunction. Similar effects are observed for other integer-spin (S = 1, 2, 3 etc.) spin states, called non-Kramers species. In many transition metal complexes zfs can be very large, on the order of 10 cm-1, which makes them "EPR-silent" at conventional (low) freqiencies and fields. In the simulation below, values of D = 10 cm-1 and E = 1.5 cm-1 were used, and a parallel orientation of the zfs tensor relative to magnetic field assumed. The red arrows represent a W-band (95 GHz) energy quantum. Even at this relatively high frequency, fields on the order of 7 T are necessary to observe any resonance. EPR resonances at X-band would be observed only very near the crossing of the MS = 0, and -1 spin levels at ca. 10.5 T.
Below is a practical application of HFEPR to overcome large zero-field splitting in an "EPR-silent" high-spin Mn(III) complex.
[Mn(dbm)2(py)2](ClO4) (dbm = anion of 1,3-diphenyl-1,3-propanedione(dibenzoylmethane), py = pyridine) is a compound of Mn(III). The spin state is a quintet (S = 2). All spin Hamiltonian parameters, including the fourth-rank terms B4i were determined from HFEPR spectra:
gx = 1.993(1), gy = 1.994(1), gz = 1.983(1)
D = – 4.504(2) cm-1 E = –0.425(1) cm-1
B40 = –1.8(4) ∙10-4 cm-1, B42 = 7(3) ∙10-4 cm-1, B44 = 48(4) ∙10-4 cm-1
The sign of the zero-field splitting parameters can be determined by analyzing the intensity pattern of a high-field spectrum because the Zeeman splitting is comparable to the thermal energy kT at low temperatures. This particular spectrum was recorded on the transmission instrument.
- Another spectrum measured on the Keck-magnet based spectrometer
- Synthesis, Crystal Structure, and High-Precision High-Frequency and -Field Electron Paramagnetic Resonance Investigation of a Manganese(III) Complex: [Mn(dbm)2(py)2](ClO4), by Aromi, G.; Telser, J.; Ozarowski, A.; Brunel, L.-C.; Stoeckli-Evans, H.-M. and Krzystek, J., Inorg. Chem., 2005, 44, 187-196.
- Numbers Useful in EPR