ENDOR and ESEEM of a Non-Kramers Doublet in an Integer-Spin System
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Spin systems with an even number of electrons are not required to exhibit degenerate levels in zero field, and many are EPR-silent. However, systems with integer total angular momentum J = L + S ≥ 2 and an appropriate type of near-axial zero-field splitting frequently are EPR active because they exhibit a quasi-degenerate, so-called non-Kramers (NK) doublet that is split in zero applied field by an energy, Δ, the average of which is within the microwave range. Recently there has been a resurgence of interest in the EPR of NK doublets because of their occurrence in a number of metallobiomolecules. However, there had been no advanced magnetic resonance studies-electron-nuclear double resonance (ENDOR) or electron spin echo envelope modulation (ESEEM)-on any non-Kramers doublet system until we examined the doubly-reduced, diiron(II) centers of two methane monooxygenase hydroxylase enzymes and of hemerythrin azide (Hoffman, B. M.; Sturgeon, B. E.; Doan, P. E.; DeRose, V. J.; Liu, K. E.; Lippard, S. J. J. Am. Chem. Soc. 1994, 116, 6023-6024). In the course of our studies, we realized that the ENDOR and electron spin-echo responses from NK doublets in general differ sharply from those of the familiar Kramers systems. The present report therefore derives the ENDOR frequencies to be expected for a nucleus coupled to a non-Kramers doublet, discusses the fundamentals of pulsed EPR, gives formulae for electron spin-echo envelope modulation (ESEEM) amplitudes, and presents examples of ESEEM time waves. © 1994 American Chemical Society.
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