Van Dyck, Wiame, Sheberstov, and Bodenhausen present an interesting study on monochromatic SLIC-based excitation of delocalized long-lived states in several molecules, with experiments performed at both low (1.4 T) and high (11.7 T) magnetic field. I found the manuscript valuable and timely, and in my opinion it is suitable for publication in Magnetic Resonance after the authors have considered the points below.

The manuscript focuses on monochromatic SLIC throughout. Since the optimization already involves varying v1, I wonder whether the authors could briefly comment on why adiabatic passage spin order conversion (APSOC) was not considered. APSOC may offer greater robustness with respect to B1​ inhomogeneity, so even a short discussion of the rationale for choosing monochromatic SLIC would strengthen the presentation.

I was also unsure how to interpret Fig. 5 quantitatively. It would be helpful if the authors stated explicitly whether the figure shows an absolute LLS conversion efficiency or a quantity normalized to a reference value. Related to this, what is the maximum achievable monochromatic SLIC efficiency under ideal conditions for spin systems containing more than two spins? In such systems the conversion efficiency is not necessarily expected to reach 100%, and I believe a short clarification would be useful for the reader.

Regarding lines 96–100, I would suggest a more careful wording. Strictly speaking, the couplings themselves are not “strong” or “weak” in isolation; rather, the relevant distinction is whether the differences in precession frequencies are large or small compared with the corresponding couplings. This terminology is straightforward in homonuclear cases, but becomes less applicable in the presence of heteronuclei inequivalently coupled to nuclei of interest.

For line 118, I would appreciate a more careful justification of the reference to a Bloch–Siegert shift. The observed shift of the non-irradiated spins under selective irradiation may instead reflect a more general AC-Zeeman-type effect. I think the manuscript would benefit from a brief clarification of the physical mechanism intended here, and why the term “Bloch–Siegert shift” is appropriate in this context. I am afraid it might not be since it involves the presence of a counter-rotating component presence of which should only be considered when B1 becomes comparable to B0.

Concerning line 127 and Eq. (3), I wondered whether other LLSs or LLCs are possible in principle for this class of systems? 

In addition, lines 129–132 and several other places would benefit from a careful stylistic pass. For example, the formatting of "J"-s is not fully consistent throughout the manuscript.

Finally, in line 148, the sentence ending with “between spins A and X denoted as …” appears to be missing the corresponding symbol or definition.



P.S. Figure 2 caption mention six molecules while I can see only 5!

I found this paper by Van Dyck and coworkers to be an enjoyable read. It is essentially the latest entry in a long line of exciting work employing variants of the popular SLIC sequence (initially introduced to singlet/LLS NMR by DeVience, Rosen, and Walsworth) to novel biomolecular applications, a "new genre" of spin choreography pioneered by Sheberstov and Bodenhausen.



It has already been shown by the present authors that SLIC may be used in AA'XX' spin systems to great effect to generate population imbalances involving the nuclear singlet state S₀; e.g.|S₀S₀>-|T₀T₀> and |S₀T₀>-|T₀S₀> (see the previous work of Warren's group). It was common sense that this tends to work neatly provided that the chemical shifts/frequencies of the A and X spins are well-separated... but it was however not so clear how well this would work in the case of poorly-separated frequencies when secularization of the AX/AX' J-coupling Hamiltonian is no longer appropriate; i.e. the AA'BB' case that one would be forced to accept in lower magnetic fields. At the time of writing the world is entering the Fifth Helium Crisis, and exploring how pulse sequence methodology works at lower magnetic fields is a highly relevant research direction if you ask me!



This work publishable in standing form subject to some minor comments:



1. There is a typo in equation 3.

2. "This means there is a “blind spot” where excitation of LLS cannot be achieved, at least not starting with high-field SLIC parameters." It may be worth pointing out that this "blind spot" occurs at an offset of sqrt(3)*nu_{SLIC}, at least in the limit of a small Delta_J. This corresponds to a condition when the off-resonance spin pairs, which ideally shouldn't be touched at all, are oscillating at a Rabi frequency twice that of nu_{SLIC}.