A simple method for hyperfine-selective heteronuclear pulsed ENDOR via proton suppression

A major reason for the importance of ENDOR spectroscopy ( 1) in the study of metal complexes is that the technique is inherently broad-banded: all coupled nuclei having spin I > 0 can be detected with comparable sensitivity (2). Unfortunately, at X-band magnetic fields ( -0.3 T at g = 2) the proton ENDOR spectrum often overlaps with and obscures the spectra of important heteronuclei such as 14N, 13C 57Fe or 33S. One solution is to work at higher microwave frequency (2, 3). In adiitioi, three elegant, two-dimensional pulsed-ENDOR techniques (4) have been developed to address this problem by hyperfine selection at X band (57). We now describe a simple one-dimensional pulsed-ENDOR method for hyperfine selection of heteronuclear ENDOR signals through the suppression of the proton pattern. As the technique also leads to enhancement of the intensities of heteronuclear signals, we call it POSHEENDOR (proton suppression, heteronuclear enhancement). Two of the currently available pulsed-ENDOR techniques for hyperiine selection are formally equivalent and can be loosely grouped as ELDOR-ENDOR experiments. Buhlmann et al. (5) used field jumps within a Davies ENDOR sequence (4, 8) to achieve selection, whereas Thomann and Bernard0 (6) used microwave frequency jumps. However, these methods require two-dimensional experiments to obtain a complete hyperfme-selected spectrum and thus need large blocks of time for data acquisition. In addition, they require equipment beyond that necessary for ordinary Davies ENDOR. The third technique, developed by de Beer et al. ( 7)) utilizes a Mims ENDOR sequence (4, 9). In this method, hyperfine selection is achieved by systematic variation of time between the first and second microwave pulses in a stimulated-echo sequence. This procedure is restricted by the time required for a 2D experiment as well as by limitation of the Mims sequence to smaller hypetine couplings. The hyperfine selection technique we describe employs the fact that in a Davies ENDOR sequence,