We thank Dr. Fabian Hecker for his constructive feedback.

L32: Thank you. We will revise this section to include the central blind spot behaviour of the Davies ENDOR sequence due to the excitation pulse length.



L120: We will include a table with hyperfine (& quadrupole) couplings of the relevant coupled nuclei observed in the ENDOR experiments to the supporting information.



Figure 2:

• We would prefer to keep the 500 W single frequency ENDOR spectrum in the figure to clearly show that higher sensitivity can be achieved with much lower power by chirp RF pulses.
• Peak intensities are compared in these panels. We will clarify this upon revision in the figure legend.
• We apologize that the position/color of the pale arrows is swapped in panel 2b) and will update this in the revision.

L186 & Figure 3: Thank you for this comment. As noted, in the figure we show that a convolution-based approach is valid to simulate the chirp ENDOR spectrum. This is most apparent by comparing with a convoluted experimental single frequency spectrum rather than a convoluted frequency domain ENDOR simulation of the spectrum because this provides the most accurate comparison by overlaying experimental data with RF-excitation width broadening together with single-frequency data broadened in post processing. Accordingly, a frequency domain simulation which describes the single frequency spectrum well, will also describe the chirp ENDOR spectrum well after convolution (note, not necessarily the other way around).

To strengthen this point, we will emphasize in the revision that simulation-based spectral analysis can be used for chirp ENDOR data in an analogous manner as for single frequency ENDOR, just with convolution as an additional step as shown in the figure.

L130: Thank you for noticing the typo.

We thank Anonymous Referee 1 for the appreciation of our work and the detailed comments. Our point-by-point answers to the comments are:

• Page 3 phase cycle: Thank you for noting this and we apologize for the missing description. The 4-step phase cycles used in the experiments are:
  
  Mims: π/2 ( 0, π, 0, π) – π/2 (0, 0, π, π) – p_RF (0, 0, 0, 0) - π/2 (0, 0, 0, 0) – Det. (1, -1, -1, 1)
  
  Davies: π ( 0, 0, 0, 0) – p_RF (0, 0, 0, 0) – π/2 (0, 0, π, π) - π (0, π, 0, π) – Det. (1, -1, -1, 1)

            We will include the phase cycles in section 2.2 of the revised manuscript.

• Page 7 Setting up the chirp pulse: Thank you for this important comment.
  
  The first parameter to choose is the bandwidth of the chirp pulse and therefore the resolution desired in the experiment (e.g. 1 MHz chirp bandwidth gives approximately 1 MHz potential resolution in the ENDOR spectrum as visible in Figs. 3 and S2). As a second step the pulse length should be chosen such that the spectral power density is high enough to achieve full inversion for all couplings with the selected chirp bandwidth. This depends on the available RF amplifier output power, the ENDOR resonator and their frequency responses. In our case, for a 100 W RF amplifier output power and a Bruker X-band MD4 ENDOR resonator an RF pulse length of ca. 100 µs was sufficient for maximum sensitivity and full inversion (see Fig. 2d). Note that further increase of the RF pulse length maintains the ENDOR sensitivity (Fig. 2d).
  
  As we fully agree with the reviewer that the experiments should be as accessible as possible, we will include a new SI section 2 “Setup and Optimization of RF chirp pulses in ENDOR experiments” in the revised version of the manuscript.
• Page 7, lines 148 – 149: We apologize for the ambiguous sentence. We wanted to convey that RF pulses longer than 100 µs do not change the ENDOR intensity anymore and are therefore not needed. We will change l148 – 149 accordingly upon revision: “Figure 2c and d show that for a chirp bandwidth of 1 MHz a pulse length of about 100 µs is sufficient to achieve maximum intensity.”

• P8, line 172: For the simulation a Gaussian distribution of purely isotropic hyperfine couplings was used. Each coupling in this distribution is described by a delta peak with an infinitesimally small linewidth. The maximum of this distribution (“mean”) was set to 4 MHz. The width of this Gaussian distribution (FWHM) is 1.2 MHz corresponding to a standard deviation of 0.5 MHz as introduced in methods section 2.3. We will clarify this, by adding “isotropic” hyperfine coupling to the explanation in 3.3, and phrase the description as the “mean of the distribution of isotropic hyperfine couplings”.
• Fig 3B: For the ENDOR simulations using convolution (Fig. 3b) the chirp pulse excitation profile is calculated from pulse length and amplitude/power with EasySpin, as we will describe in more detail in the methods section 2.3 and in the SI section 1. This pulse excitation profile (as function of frequency offset from chirp center frequency) is convoluted with the experimental single frequency ENDOR spectrum. For full transparency and for those interested in the script of this simulation, we refer to the data and script accompanying this manuscript on zenodo 10.5281/zenodo.11082486.
• TRIPLE: We agree that a demonstration on different systems (e.g. including organic radicals) will be useful to get a better understanding of the performance of TRIPLE on diverse paramagnetic systems, however, for the application class of metal sites (as e.g. in catalysis) CuTPP is a relevant model system and hence test case. Thus, for applications in fields as catalysis, material science and bioinorganic chemistry, the chirp TRIPLE experiment on CuTPP in this paper showcases the utility of chirp pulses in 2D experiments. However, we do agree with the reviewer that on other systems such as slow-relaxing organic radicals, the performance of TRIPLE may well be somewhat worse, as also the case for TRIPLE without chirp pulses – however, this is beyond the scope of this paper.
• P2 line 7: We thank the reviewer for noting this incorrect citation, we will correct this in the revised manuscript.
  
  
  
  

We thank Anonymous Referee 2 for the nice feedback and the detailed comments. Our answers to the comments are:

• We will add the experimental EPR spectrum to the supporting information and indicate the field position used for ENDOR, along with a table of the known spin Hamiltonian parameters for overview.
• We apologize for sharing the wrong zenodo link in the reply to Anonymous Referee 1. The correct zenodo doi is 5281/zenodo.13625320. Also, we will update the experimental section with a more thorough explanation of the ENDOR simulations.
• What we meant with the simulations “become infeasible” is that the computational cost to do time domain simulations for spin systems with a high number of spins increases exponentially, thus hampering spectral analysis by least-squares fitting of spin Hamiltonian parameters. This is the reason, why we think the convolution approach is of interest for the typical ENDOR spectroscopist: One can still use frequency domain simulations (as implemented in EasySpin) to analyze the experimental chirp ENDOR spectra as described. Regarding spin dynamics simulations of 14N: Thank you for pointing this out. We will consider this in the revision.

• We apologize for writing this not clearly enough. We did not to compare chirp ENDOR with special TRIPLE experiments, but just point out to the reader that in contrast to the chirp ENDOR experiments, in special TRIPLE experiments the excitation of 2 NMR transitions is exploited in a favorable, quantitative fashion. We will clarify this in the revision.
• We agree that it is in the hands of the operator to adjust the RF power carefully. Nevertheless, if samples have low signal intensities, sometimes one may choose higher sensitivities with shorter/stronger RF pulses at the compromise of introducing amplifier overtones. As explained in the manuscript, chirp pulses with lower power can be superior to single frequency pulses with high power and we encourage spectroscopists to use chirp pulses without introducing overtones in the spectrum. We will clarify this in the revision of the manuscript.
• As already mentioned in point 1 and the reply to the community comment by Fabian Hecker, we will include a table with hyperfine couplings for Cu-TPP in the manuscript. We will annotate the 2D TRIPLE spectrum with hyperfine and nuclear quadrupolar couplings. We agree that for CuTPP it is enough to record 1D TRIPLE traces at a few hfcs to read out the same information as from the 2D TRIPLE. However, this is a demonstration for more complicated systems with multiple paramagnetic centres, where 2D TRIPLE is required for a clear interpretation and we show (although on the comparably simple CuTPP system) that chirp RF pulses are valuable also for such cases.
• We will include the measurement (30 min/TRIPLE trace) in the figure caption. Please be aware that the 2D TRIPLE was measured with a slightly different number of points and non-uniform sampling.
• Thank you for the comment. We expect that the bandwidth-dependent effects in TRIPLE will be similar to ENDOR results as long as the chirp does not excite multiple transitions, and we therefore did not investigate this further. We agree that for 2D TRIPLE with maximum sensitivity the chirp pulse bandwidth should be adjusted to the width of the anisotropic line, as we note in the text, the chirp bandwidth could even be adapted specifically to the width of each ENDOR line, resulting in a non-uniform bandwidth excitation scheme.