Double Nuclear Coherence Transfer (DONUT)-HYSCORE:  A New Tool for the Assignment of Nuclear Frequencies in Pulsed EPR Experiments

A two-dimensional experiment, termed DONUT-HYSCORE ( do uble nu clear coherence t ransfer hy perfine s ublevel cor relation) designed to obtain correlations between nuclear frequencies belonging to the same electron spin manifold is presented. The sequence employed is π/2−τ 1 −π/2− t 1 −π−τ 2 −π− t 2 −π/2−τ 1 −echo, and the echo is measured as a function of t 1 and t 2 whereas τ 1 and τ 2 are held constant. It is complementary to the standard HYSCORE experiment which generates correlations between nuclear frequencies belonging to different M S manifolds and is particularly useful for 14 N nuclei. The experiment is first demonstrated on a single crystal of copper-doped l -histidine hydrochloride monohydrate where the modulations are induced by a single 14 N nucleus, the remote nitrogen in the imidazole group. HYSCORE and DONUT-HYSCORE experiments were carried out on two crystal orientations. In the first, one Cu 2+ site contributes to the echo and all six nuclear frequencies together with the expected correlation were observed. In the second, 12 frequencies corresponding to two Cu 2+ ions at different crystallographic sites appeared and all expected correlations were detected as well. This rather trivial example demonstrates that the DONUT-HYSCORE pulse sequence indeed generates correlations within the M S manifolds. The value of the DONUT-HYSCORE experiment is demonstrated on a frozen solution of a vanadyl complex with a bis-hydroxamate ion binder (VO-RL515). The modulations in this complex arise from the two 14 N nuclei in the hydroxamate groups, and orientation-selective three-pulse ESEEM ( e lectron s pin− e cho e nvelope m odulation) spectra showed a number of well-resolved peaks. An unambiguous assignment of all peaks and their orientation dependences could not be achieved through HYSCORE alone because at certain orientations frequencies of one of the M S manifolds were absent or overlapped with those of the other manifold. The application of the DONUT-HYSCORE experiment provided new correlations that led to the complete assignment of the ESEEM frequencies, thus paving the way for future systematic spectral simulations for the determination of the best-fit Hamiltonian parameters. This example shows that, in the case that the HYSCORE experiment cannot distinguish between two sets of frequencies belonging to the same M S manifold in different centers (or orientations) because signals from the other manifold are missing or overlapping, the DONUT-HYSCORE becomes most valuable.