Two-dimensional pulsed TRIPLE at 95 GHz

The one-dimensional (1D) pulsed TRIPLE resonance experiment, introduced by Mehring et al. (M. Mehring, P. Hofer, and A. Grupp, Ber. Bunseges. Phys. Ch em. 91, 1132-1137 (1987)) is a modification of the standard Davies ENDOR ex periment where an additional RF pi-pulse is applied during the mixing time. While the first RF pulse is set to one of the ENDOR transitions, the frequ ency of the second RF pulse is scanned to generate the TRIPLE spectrum. The difference between this spectrum and the ENDOR spectrum yields the differe nce TRIPLE spectrum, which exhibits only ENDOR lines that belong to the sam e M-S manifold as the one selected by the first RF pulse. We have extended this experiment in two dimensions (2D) by sweeping the frequencies of both RF pulses. This experiment is particularly useful when the spectrum is cong ested and consists of signals originating from different paramagnetic cente rs. The connectivities between the peaks in the 2D spectrum enable a straig htforward assignment of the signals to their respective centers and M-S man ifolds, thus providing the relative signs of hyperfine couplings. Carrying out the experiment at high fields has the additional advantage that nuclei with different nuclear gyromagnetic ratios are well separated. This is part icularly true for protons which appear at significantly higher frequencies than other nuclei. The feasibility and effectiveness of the experiment is d emonstrated at W-band (94.9 GHz) on a crystal of Cu2+-doped L-histidine. Ho monuclear H-1-H-1, N-14/Cl-35-N-14/Cl-35 and heteronuclear H-1-N-14/Cl-35 2 D TRIPLE spectra were measured and from the various connectivities in the 2 D map the H-1, N-14 and Cl-35 signals that belong to two different Cu2+ cen ters were identified and grouped according to their M-S manifolds. (C) 2000 Academic Press.