Abstract
A new ultra-wide band (UWB) pulse EPR method is introduced for observing all nuclear frequencies of a paramagnetic center in a single shot. It is based on burning spectral holes with a high turning angle (HTA) pulse that excites forbidden transitions and subsequent detection of the hole pattern by a chirp echo. We term this method Chirp Echo Epr SpectroscopY (CHEESY)-detected NMR. The approach is a revival of FT EPR-detected NMR. It yields similar spectra and the same type of information as electron-electron double resonance (ELDOR)-detected NMR, but with a multiplex advantage. We apply CHEESY-detected NMR in Q band to nitroxides and correlate the hyperfine spectrum to the EPR spectrum by varying the frequency of the HTA pulse. Furthermore, a selective π pulse before the HTA pulse allows for detecting hyperfine sublevel correlations between transitions of one nucleus and for elucidating the coupling regime, the same information as revealed by the HYSCORE experiment. This is demonstrated on hexaaquamanganese(II). We expect that CHEESY-detected NMR is generally applicable to disordered systems and that our results further motivate the development of EPR spectrometers capable of coherent UWB excitation and detection, especially at higher fields and frequencies.
Highlights
Hyperfine couplings and nuclear frequencies can give detailed and valuable information about the local structure of paramagnetic centers
We demonstrate the use of chirp echoes to detect the complete Fourier transform (FT) EPR-detected NMR spectrum of a nitroxide radical and the correlation to the EPR spectrum by changing the frequency of the high turning angle (HTA) pulse step by step
Note that chirp echoes can be used to detect any change in polarization, the ones associated with hyperfine spectroscopy
Summary
Hyperfine couplings and nuclear frequencies can give detailed and valuable information about the local structure of paramagnetic centers. Jeschke / Journal of Magnetic Resonance 289 (2018) 26–34 couplings and broad holes, because of the limited excitation bandwidth and severe dead time Both of these limitations can be overcome by the use of chirp echoes, where the frequency is swept through the spectrum during the pulse [10]. The ESEEM experiment suffers from short transverse relaxation times typical for systems with medium to large couplings, and from transverse interference effects [12] In our hands, it did not yield satisfying spectra for disordered systems, most likely due to destructive interference from closely spaced nuclear frequencies with different phases. We demonstrate the use of chirp echoes to detect the complete FT EPR-detected NMR spectrum of a nitroxide radical and the correlation to the EPR spectrum by changing the frequency of the HTA pulse step by step. A hyperfine sublevel correlation (HYSCORE)-type correlation experiment is introduced that is based on a polarization transfer step by a p-pulse on an allowed transition before the HTA pulse
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