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 (double nuclear coh erence transfer hyperfine sublevel correlation) designed to obtain cor relations between nuclear frequencies belonging to the same electron s pin manifold is presented. The sequence employed is au(1)-pi/2-t(1)-pi -tau(2)-pi-t(2)-pi/2-tau(1)-echo and the echo is measured as a functio n of t(1) and t(2) whereas tau(1) and tau(2) are held constant. It is complementary to the standard HYSCORE experiment which generates corre lations between nuclear frequencies belonging to different Ms manifold s and is particularly useful for N-14 nuclei. The experiment is first demonstrated on a single crystal of copper-doped E-histidine hydrochlo ride monohydrate where the modulations are induced by a single N-14 nu cleus, the remote nitrogen in the imidazole group. HYSCORE and DONUT-H YSCORE experiments were carried out on two crystal orientations. In th e first, one Cu2+ Site contributes to the echo and all six nuclear fre quencies together with the expected correlation were observed. In the second, 12 frequencies corresponding to two Cu2+ ions at different cry stallographic sites appeared and all expected correlations were detect ed as well. This rather trivial example demonstrates that the DONUT-HY SCORE pulse sequence indeed generates correlations within the Ms manif olds. The value of the DONUT-HYSCORE experiment is demonstrated on a f rozen solution of a vanadyl complex with a bis-hydroxamate ion binder (VO-RL515), The modulations in this complex arise from the two N-14 nu clei in the hydroxamate groups, and orientation-selective three-pulse ESEEM (electron spin-echo envelope modulation) spectra showed a number of well-resolved peaks. An unambiguous assignment of all peaks and th eir orientation dependences could not be achieved through HYSCORE alon e because at certain orientations frequencies of one of the M-S manifo lds were absent or overlapped with those of the other manifold. The ap plication of the DONUT-HYSCORE experiment provided new correlations th at led to the complete assignment of the ESEEM frequencies, thus pavin g the way for future systematic spectral simulations for the determina tion 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 c enters (or orientations) because signals from the other manifold are m issing or overlapping, the DONUT-HYSCORE becomes most valuable.