UWB DEER and RIDME distance measurements in Cu(II)–Cu(II) spin pairs

Abstract

Distance determination by Electron Paramagnetic Resonance (EPR) based on measurements of the dipolarcoupling are technically challenging for electron spin systems with broad spectra due to comparativelynarrow microwave pulse excitation bandwidths. With Na4[{Cu^II(PyMTA)}-(stiff spacer)-{Cu^II(PyMTA)}]as a model compound, we compared DEER and RIDME measurements and investigated the use offrequency-swept pulses. We found very large improvements in sensitivity when substituting themonochromatic pump pulse by a frequency-swept one in DEER experiments with monochromatic obser-ver pulses. This effect was especially strong in X band, where nearly the whole spectrum can be includedin the experiment. The RIDME experiment is characterised by a trade-off in signal intensity and modula-tion depth. Optimal parameters are further influenced by varying steepness of the background decay. Asimple 2-point optimization experiment was found to serve as good estimate to identify the mixing timeof highest sensitivity. Using frequency-swept pulses in the observer sequences resulted in lower SNR inboth the RIDME and the DEER experiment. Orientation selectivity was found to vary in both experimentswith the detection position as well as with the settings of the pump pulse in DEER. In RIDME, orientationselection by relaxation anisotropy of the inverted spin appeared to be negligible as form factors remainrelatively constant with varying mixing time. This reduces the overall observed orientation selection tothe one given by the detection position. Field-averaged data from RIDME and DEER with a shaped pumppulse resulted in the same dipolar spectrum. We found that both methods have their advantages and dis-advantages for given instrumental limitations and sample properties. Thus the choice of method dependson the situation at hand and we discuss which parameters should be considered for optimization.