ENDOR AND ESEEM OF A NON-KRAMERS DOUBLET IN AN INTEGER-SPIN SYSTEM

Spin systems with an even number of electrons are not required to exhi bit degenerate levels in zero field, and many are EPR-silent. However, systems with integer total angular momentum J = L + S greater than or equal to 2 and an appropriate type of near-axial zero-field splitting frequently are EPR active because they exhibit a quasidegenerate, so- called non-Kramers (NK) doubler that is split in zero applied field by an energy, Delta, the average of which is within the microwave range. Recently there has been a resurgence of interest in the EPR of NK dou blers because of their occurrence in a number of metallobiomolecules. However, there had been no advanced magnetic resonance studies-electro n-nuclear double resonance (ENDOR) or electron spin echo envelope modu lation (ESEEM)-on any non-Kramers doublet system until we examined the doubly-reduced, diiron(II) centers of two methane monooxygenase hydro xylase enzymes and of hemerythrin azide (Hoffman, B. M.; Sturgeon, B. E.; Dean, P. E.; DeRose, V. J.; Liu, K. E.; Lippard, S. J. J. Am. Chem . Sec. 1994, 116, 6023-6024). In the course of our studies, we realize d that the ENDOR and electron spin-echo responses from NK doublers 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 doubler, discusses the fundame ntals of pulsed EPR, gives formulae for electron spin-echo envelope mo dulation (ESEEM) amplitudes, and presents examples of ESEEM time waves .