Preprints
https://doi.org/10.5194/mr-2024-14
https://doi.org/10.5194/mr-2024-14
10 Sep 2024
 | 10 Sep 2024
Status: this preprint is currently under review for the journal MR.

Increased sensitivity in Electron Nuclear Double Resonance spectroscopy with chirped radiofrequency pulses

Julian Stropp, Nino Wili, Niels Christian Nielsen, and Daniel Klose

Abstract. Electron Nuclear Double Resonance (ENDOR) spectroscopy is an EPR technique to detect the nuclear frequency spectra of hyperfine coupled nuclei close to paramagnetic centres, which have interactions that are not resolved in continuous wave EPR spectra and may be fast relaxing on the time scale of NMR. For the common case of non-crystalline solids, such as powders or frozen solutions of transition metal complexes, the anisotropy of the hyperfine and nuclear quadrupole interactions renders ENDOR lines often several MHz broad, thus diminishing intensity. With commonly used ENDOR pulse sequences only a small fraction of the NMR/ENDOR line is excited with a typical RF pulse length of several tens of μs, and this limits the sensitivity in conventional ENDOR experiments. In this work, we show the benefit of chirped RF excitation in frequency domain ENDOR as a simple yet effective way to significantly improve sensitivity. We demonstrate on a frozen solution of Cu(II)-tetraphenylporphyrin that the intensity of broad copper and nitrogen ENDOR lines increases up to 9-fold compared to single frequency RF excitation, thus making the detection of metal ENDOR spectra more feasible. The tunable bandwidth of the chirp RF pulses allows the operator to optimize for sensitivity and choose a tradeoff with resolution, opening up options previously inaccessible in ENDOR spectroscopy. Also, chirp pulses help to reduce RF amplifier overtones, since lower RF powers suffice to achieve intensities matching conventional ENDOR. In 2D TRIPLE experiments the signal increase exceeds 10 times for some lines, thus making chirped 2D TRIPLE experiments feasible even for broad peaks in manageable acquisition times.

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Julian Stropp, Nino Wili, Niels Christian Nielsen, and Daniel Klose

Status: open (until 18 Oct 2024)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • CC1: 'Comment on mr-2024-14', Fabian Hecker, 02 Oct 2024 reply
    • AC1: 'Reply on CC1', Daniel Klose, 07 Oct 2024 reply
  • RC1: 'Comment on mr-2024-14', Anonymous Referee #1, 04 Oct 2024 reply
    • AC2: 'Reply to RC1', Daniel Klose, 08 Oct 2024 reply
Julian Stropp, Nino Wili, Niels Christian Nielsen, and Daniel Klose
Julian Stropp, Nino Wili, Niels Christian Nielsen, and Daniel Klose

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Short summary
Sensitivity is often the limiting factor in ENDOR. Here, we demonstrate how using chirp radiofrequency pulses can improve ENDOR sensitivity up to 3-9-fold, with the strongest increase for broader lines often encountered in disordered solids for nuclei such as nitrogen and metals. The resulting drastic speed-up in acquisition times renders also 2D ENDOR more feasible, as we demonstrate in 2D TRIPLE showing correlations of Cu hyperfine couplings.