Frequency swept pulses for the enhanced resolution of ENDOR spectra detecting on higher spin transitions of Gd(III)

Half-Integer High Spin (HIHS) systems with zero-field splitting (ZFS) parameters below 1 GHz are generally dominated by the spin |─1/2>→|+1/2> central transition (CT). Accordingly, most pulsed Electron Paramagnetic Resonance (EPR) experiments are performed at this position for maximum sensitivity. However, in certain cases it can be desirable to detect higher spin transitions away from the CT in such systems. Here, we describe the use of frequency swept Wideband, Uniform Rate, Smooth Truncation (WURST) pulses for transferring spin population from the CT, and other transitions, of Gd(III) to the neighbouring higher spin transition |─3/2>→|─1/2> at Q- and W-band frequencies. Specifically, we demonstrate this approach to enhance the sensitivity of 1H Mims Electron-Nuclear Double Resonance (ENDOR) measurements on two model Gd(III) aryl substituted 1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid (DO3A) complexes, focusing on transitions other than the CT. We show that an enhancement factor greater than 2 is obtained for both complexes at Q- and W-band frequencies by the application of two polarising pulses prior to the ENDOR sequence. This is in agreement with simulations of the spin dynamics of the system during WURST pulse excitation. The technique demonstrated here should allow more sensitive experiments to be measured away from the CT at higher operating temperatures, and be combined with any relevant pulse sequence.