Simulating Suppression Effects in Pulsed ENDOR, and the 'Hole in the Middle' of Mims and Davies ENDOR Spectra

All pulsed electron-nuclear double resonance (ENDOR) techniques, and in particular the Mims and Davies sequences, suffer from detectability biases ('blindspots') that are directly correlated to the size of the hyperfine interactions of coupled nuclei. Our efforts at ENDOR 'crystallography' and 'mechanism determination' with these techniques have led our group to refine our simulations of pulsed ENDOR spectra to take into account these biases, and we here describe the process and illustrate it with several examples. We first focus on an issue whose major significance is not widely appreciated, the 'hole in the middle' of pulsed ENDOR spectra caused by the n = 0 suppression hole in Mims ENDOR and by the analogous A -> 0 suppression in Davies ENDOR for I = A1/2 and for H-2 (I = 1). We then discuss the general treatment of suppression effects for I = 1, illustrating it with a treatment of Mims suppression for N-14.