Hyperfine decoupling in electron spin resonance

A new class of experiments is introduced to electron spin resonance (ESR) spectroscopy that utilizes hyperfine decoupling for resolution enhancement and spectrum simplification, and that provides a basis for correlation techniques. A general framework is provided for the discussion of pulse ESR experiments on systems with arbitrary effective electron spin S and an arbitrary number of coupled nuclear spins and is used to describe spin decoupling in pulse ESR and ENDOR spectroscopy. Analytical expressions are given for the hyperfine-decoupled nuclear frequencies and the residual hyperfine splittings of spin-1/2 nuclei during strong decoupling. Pulse sequences are proposed for hyperfine-decoupled electron spin echo envelope modulation (ESEEM) and electron nuclear double resonance (ENDOR) experiments as well as for the correlation of the hyperfine-decoupled ESEEM spectrum with two-pulse and three-pulse ESEEM spectra and of hyperfine-decoupled ENDOR with the hyperfine splittings. It is shown that hyperfine-decoupled ESEEM and ENDOR spectra can reveal information on the magnetic quantum numbers involved in an ESR observer transition, and that choosing a transition mS↔mS+1 with mS≠−1/2 can improve the resolution of a nuclear frequency spectrum. In addition, such experiments can be used to determine the relative signs of hyperfine couplings. The potential of the two-dimensional DECENT (decoupled ESEEM correlated to nuclear transition frequencies) experiment is demonstrated on weakly coupled N14 nuclei in both an ordered and a disordered system and on the hexaquo manganese (II) complex (S=5/2) in a single crystal. It is also shown that for the ESR observer transition mS=(−3/2↔−5/2) the S=5/2 system yields highly resolved hyperfine-decoupled ENDOR spectra which allow for a complete assignment of the ENDOR lines.