Pulse endor

Abstract

Abstract Apart from the ESEEM methods described in the previous two chapters, electron nuclear double resonance (ENDOR) spectroscopyl ‘l is the other well-established magnetic resonance technique for measuring nuclear transition frequencies of paramagnetic compounds. In this chapter, we describe the basic principles of pulse ENDOR and examine in which situations it is better suited than c.w. ENDOR or ESEEM to obtain nuclear frequency spectra (§12.1). We then discuss in some detail the two standard pulse schemes, Davies and Mims ENDOR, which are based on polarization transfers by r.f. pulses(§12.2). By extending these schemes to triple resonance, relative signs of hyperfine couplings can be determined and nuclear frequencies of the same site in a crystal or same orientation in a disordered system can be correlated. Another simple extension, multiple quantum ENDOR, can be used to estimate the number of coupled nuclei. ENDOR schemes based on coherence transfer by r.f. pulses, in particular, hyperfinedecoupled ENDOR are discussed in §12.3. We then examine in which cases time-domain ENDOR schemes provide a resolution enhancement or can be used in 2D correlation experiments (§12.4). Finally the correlation of nuclear frequency spectra to the more simple hyperfine spectra is discussed as a strategy for increasing both resolution and information