Review Reports - Anti-Electrostatic Anion-Anion Noncovalent Interactions Are Not Halogen Bonds: Evidence from X···O Contacts in XO<sub>4</sub><sup>−</sup> Dimers and Oligomers in Crystals Structures

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The paper Anti-Electrostatic Anion-Anion Noncovalent Interactions Are Not Halogen Bonds: Evidence from X···O Contacts in XO₄ ⁻ Dimers and Oligomers in Crystals Structures reports on the computational study of perhalate anions and the nature of anion-anion interactions within dimers and oligomers of XO₄^–. The study combines a number of computational methods including The Molecular Electrostatic Surface Potential (MESP), the Quantum Theory of Atoms in Molecules (QTAIM) and the Independent Gradient Model based on Hirshfeld partitioning (IGMH). The paper objective is clearly stated, Introduction is concise and relevant and Figures are of excellent quality and very informative. The results are also clearly presented with in-depth result analysis and discussion. I strongly suggest the publication of this paper in International Journal of Molecular Sciences. Below is the short list of questions/comments that should be addressed to Authors.

According to the IJMS Instructions for Authors, the article should contain a Results section. In this Article, the results appear to be distributed across Sections 3–6 rather than presented in a clearly stated Results section.

In addition, I find the purpose of Section 2 somewhat unclear. It does not seem to function as a standard Introduction, and does not present the results of this work. I suggest to incorporate this section into 1. Introduction.

The almost identical paragraph appear two times in a row – 1. lines 160-168; 2. lines 169-177. Authors must decide which one to use.

Table 1 – font typo; typo: line 187 …in figure 1(i)

Author Response

Reply to Reviewer 1

Reply: We sincerely thank the Reviewer for the careful evaluation of our manuscript and for the highly positive and encouraging comments. We are particularly grateful for the Reviewer’s appreciation of the clarity of the manuscript objectives, the conciseness of the Introduction, the quality of the figures, and the depth of the analysis and discussion. We also greatly appreciate the Reviewer’s recognition of the computational methodology employed in this work, including the MESP, QTAIM, and IGMH analyses.

We are especially thankful for the Reviewer’s strong recommendation for publication in the International Journal of Molecular Sciences. The constructive comments and questions provided by the Reviewer have helped us improve the quality and clarity of the manuscript further. Our detailed responses to each point are provided below.

According to the IJMS Instructions for Authors, the article should contain a Results section. In this Article, the results appear to be distributed across Sections 3–6 rather than presented in a clearly stated Results section.

Reply: We thank the Reviewer for this important suggestion. Following the recommendation, we have now introduced a dedicated “Results” section, under which the previously existing sections are reorganized as subsections (Sections 3.1, 3.2, etc.) to improve the overall structure and compliance with the IJMS formatting guidelines.

Regarding Section 2, we agree that its role required further clarification. However, instead of merging it into the Introduction, which would substantially increase the length and reduce the readability of the introductory discussion, we retained it as a separate section. To clarify its purpose, we have now added an explanatory statement at the end of the Introduction indicating that Section 2 provides essential conceptual background of halogen bonding intended to help the reader better understand the structural features and terminology before the presentation of the main results.

The almost identical paragraph appears two times in a row – 1. lines 160–168; 2. lines 169–177. Authors must decide which one to use.

Reply: We apologize for the repetition. The duplicated paragraph has now been removed from the revised manuscript.

Table 1 – font typo; typo: line 187 …in figure 1(i).

Reply: We thank the Reviewer for identifying these typographical issues. The formatting inconsistency in Table 1 and the typo in “Figure 1(i)” have now been corrected in the revised manuscript.

We hope that the revisions and clarifications provided satisfactorily address the Reviewer’s comments and further improve the quality and clarity of the manuscript.

Reviewer 2 Report

Comments and Suggestions for Authors

The manuscript by Varadwaj and co-authors reports a computational study of anion–anion noncovalent interactions in perhalate crystal structures. The study pointed out that the frequently reported X···O contacts between XO4⁻ anions (X = Cl, Br, I) should not be classified as halogen bonds. The authors demonstrate that isolated XO4⁻ anions lack electrophilic σ-holes, that gas-phase anion–anion assemblies are intrinsically repulsive, and that the observed structural motifs are instead stabilized by solvent screening and crystal packing forces.

Overall, the study is well organized, and the use of complementary computational approaches including MESP, QTAIM, IGMH, NBO, and SAPT provides a coherent and multi-layered dataset supporting the main conclusions. The clarification of anti-electrostatic anion–anion interactions is particularly timely and could be significant for the broader field of noncovalent interactions and crystal engineering. Overall, the study addresses an important and currently debated topic in noncovalent interaction chemistry. However, several methodological and conceptual issues need to be resolved before the manuscript is suitable for publication:

The authors build their entire argument around the absence of an electrophilic σ-hole, but simultaneously discuss "negative σ-holes" as directional features throughout Section 3. Is this genuinely contradictory? if a negative region can still drive directional bonding (as shown in Figure 1), then the absence of a positive σ-hole alone can't be the definitive criterion for ruling out halogen bonding.
If σ-holes on chlorine only appear when you change the isodensity surface from 0.001 to 0.0015 a.u., then the conclusion that they are "absent" becomes surface-dependent rather than physically definitive. The authors actually acknowledge this themselves (lines 256–260) but don't follow through on the implications, which is exactly what makes this comment so pointed. It's not a peripheral methodological concern, it directly undermines the key conclusion.
Can the authors elaborate on why water? specifically, and have they tested sensitivity to the dielectric constant? The perhalate anions are studied in crystal structures, yet the calculations use water as the implicit solvent, which is an extremely polar environment. This likely over-screens coulombic repulsion and artificially stabilizes the dimers. A less polar medium would probably reduce this effect and could even change the sign of the binding energies.
Some comments on the structure of the manuscript: 1. The abstract is quite dense and could be trimmed, it essentially reproduces the discussion. 2. The conclusion reads more like a second discussion section than a conclusion. A good conclusion here should be 5-7 sentences max: what was done, what was found, and one forward-looking sentence.

Author Response

Reply to reviewer 2

Reply: We sincerely thank the Reviewer for the careful assessment of our manuscript and for the highly constructive comments. We greatly appreciate the Reviewer’s positive evaluation of the organization of the study, the multi-faceted computational approach employed, and the significance of clarifying the nature of anti-electrostatic anion–anion interactions in perhalate systems. We are particularly encouraged by the Reviewer’s recognition that the present work addresses an important and actively debated topic in the field of noncovalent interactions and crystal engineering.

The Reviewer’s comments and suggestions have been extremely valuable in improving the clarity, rigor, and presentation of the manuscript. In the revised version, we have carefully addressed all concerns raised and introduced corresponding modifications throughout the manuscript. Detailed point-by-point responses are provided below.

The authors build their entire argument around the absence of an electrophilic σ-hole, but simultaneously discuss "negative σ-holes" as directional features throughout Section 3. Is this genuinely contradictory? if a negative region can still drive directional bonding (as shown in Figure 1), then the absence of a positive σ-hole alone can't be the definitive criterion for ruling out halogen bonding.

Reply: We thank the Reviewer for this important comment. In the systems examined here, electrophilic σ-holes on the halogen atoms of the anions are absent, although the intermolecular interactions remain directional. Our central argument is that directionality alone is insufficient to classify an interaction as a halogen bond, since negative σ-holes may also participate in directional intermolecular organization. For this reason, we explicitly introduced Section 3.1, which demonstrates the directional involvement of the negative σ-hole on F along the extension of the C–F bond in CH₃F. Furthermore, we found no evidence in solution media that the negative σ-hole transforms into an electrophilic one capable of engaging attractively with a nucleophilic site, as required for conventional halogen bonding. As discussed in Section 2, the definition of halogen bonding requires the presence of an electrophilic region on the covalently bonded halogen atom, which is not the case for the systems examined in this study, thereby we cannot claim any presence of halogen bonding in the dimers or oligomers examined.

If σ-holes on chlorine only appear when you change the isodensity surface from 0.001 to 0.0015 a.u., then the conclusion that they are "absent" becomes surface-dependent rather than physically definitive. The authors actually acknowledge this themselves (lines 256–260) but don't follow through on the implications, which is exactly what makes this comment so pointed. It's not a peripheral methodological concern, it directly undermines the key conclusion.

Reply: We thank the Reviewer for this important observation. We agree that the appearance of σ-holes may depend on the chosen isodensity surface and that the commonly used 0.001 a.u. envelope is not universally definitive, as also discussed in several of our previous studies (e.g., Crystals 2020, 10, 146;) and recent review article (e.g. Int. J. Mol. Sci.2026, 27, 3352). In the present system, the 0.001 a.u. isodensity surface does not reveal a σ-hole on the halogen center, whereas the use of a higher isodensity surface does reveal the corresponding anisotropy. Importantly, however, the revealed σ-hole remains negative rather than electrophilic. Thus, while the visibility of the anisotropy depends on the chosen isodensity envelope, its electrostatic character remains consistently negative. Since the perhalate anions examined are entirely negatively charged species, the absence of a visible σ-hole at 0.001 a.u. should not be interpreted as evidence for the existence of a positive σ-hole. Therefore, our central conclusion remains unchanged: the systems studied do not exhibit the electrophilic halogen sites required for conventional halogen bonding.

Can the authors elaborate on why water? specifically, and have they tested sensitivity to the dielectric constant? The perhalate anions are studied in crystal structures, yet the calculations use water as the implicit solvent, which is an extremely polar environment. This likely over-screens coulombic repulsion and artificially stabilizes the dimers. A less polar medium would probably reduce this effect and could even change the sign of the binding energies.

Reply: We thank the Reviewer for this important comment. Water was selected as the implicit solvent primarily to examine whether a highly polar dielectric environment is capable of sufficiently screening the strong Coulombic repulsion between the like-charged perhalate anions, thereby permitting the formation of metastable dimeric arrangements. Our intention was not to reproduce the exact dielectric environment of the crystal lattice, but rather to evaluate whether dielectric screening alone could facilitate close anion–anion association. The use of water as an implicit medium is also consistent with several previous computational discussed in a recent review (Chem. Soc. Rev., 2024,53, 6654-6674).

We agree with the Reviewer that the magnitude, and potentially even the sign, of the interaction energies may depend on the dielectric constant of the surrounding medium. In less polar environments, electrostatic repulsion is expected to be less effectively screened, which could further destabilize the dimers, although the existence of extended inorganic motifs formed by these anions in the crystalline phase suggests that additional environmental and packing effects contribute significantly to their organization. We have now clarified this point in the revised manuscript and explicitly noted that the calculated stabilization is solvent-dependent and does not necessarily reflect intrinsic gas-phase attraction between the isolated anions. A broader investigation of solvent effects on the stability of these assemblies is currently underway.

Some comments on the structure of the manuscript: 1. The abstract is quite dense and could be trimmed, it essentially reproduces the discussion. 2. The conclusion reads more like a second discussion section than a conclusion. A good conclusion here should be 5-7 sentences max: what was done, what was found, and one forward-looking sentence.

Reply: We thank the Reviewer for this valuable suggestion. We agree that the previous “Discussion and Conclusions” section was overly extensive and functioned primarily as a discussion section. Accordingly, in the revised manuscript, we have retained the detailed scientific interpretation under a separate “Discussion” section and added a concise standalone “Conclusions” section summarizing the principal findings and implications of the present study. In addition, the Abstract has been shortened and streamlined to avoid excessive overlap with the Discussion section.

Reviewer 3 Report

Comments and Suggestions for Authors

This manuscript addresses a timely and conceptually important question in supramolecular chemistry: whether directional anion-anion contacts involving perhalate anions (XO₄⁻, X = Cl, Br, I) can be legitimately classified as halogen bonds (HaBs). The topic is highly relevant, given the ongoing debate and recent expansion of the HaB concept to include anti-electrostatic and like-charge interactions. The authors employ a multi-pronged computational approach (MESP, QTAIM, IGMH, NBO, and SAPT) to argue that these contacts lack the defining electrophilic σ-hole and are instead environment-stabilized, anti-electrostatic associations. The work is a necessary and healthy critique of a burgeoning trend and is conceptually sound in its main conclusion. However, the current version suffers from significant structural, methodological, and argumentative weaknesses that prevent me from recommending acceptance.

Recommendation: Major Revision

The Fundamental Limitation of the Model: Cluster-in-Cavity vs. True Crystal Environment

The central thesis of the paper is that the observed contacts are not intrinsic to the anion pair but are a product of their crystalline environment. This conclusion is drawn from calculations on finite clusters (dimers, oligomers) extracted from the CSD and treated with an implicit solvent model (SMD). This approach contains a paradox: the very environment the authors claim is responsible for the stabilization is stripped away and replaced by a structureless dielectric continuum. The authors themselves note a catastrophic artifact for the (BrO₄⁻)₂ dimer, where the binding energy is "unrealistically large." This single failure, rather than being an exception, casts significant doubt on the reliability of the SMD model for all such highly charged systems, as it can artificially over-screen or under-screen Coulombic repulsion in an uncontrolled manner. To make their conclusion truly convincing, the authors must demonstrate that the environment stabilizes the contact in the real crystal. The only rigorous way to do this is through periodic boundary condition (PBC) DFT calculations (e.g., using open source CP2K) on the full experimental unit cell.

Structural and Stylistic Issues

The manuscript's current structure obscures its message. Specifically, two sections act as lengthy tangents that disrupt the narrative flow: Section 3 (on CH₃F complexes) is an elaborate discussion of negative σ-holes in neutral molecules. While chemically interesting, it does not serve as a model for the highly charged XO₄⁻ systems and should be removed or condensed into a single sentence. Section 7 (on solvent-mediated attraction in neutral molecules) broadens the scope to anti-electrostatic interactions in neutral dimers. The conceptual parallel is clear, but its placement after the extensive and conclusive SAPT/QTAIM analysis of the anion dimers and immediately before the Conclusion is structurally counterproductive. It introduces a new set of model systems at a point where the reader expects a final resolution of the main anion problem. If the authors wish to retain this discussion, it would be far more effective as a bridge between the theoretical definitions (Section 2) and the main results (Section 5), or alternatively as a concise comparative paragraph within the Discussion.

CSD Analysis: The selection of structures appears subjective. A systematic, script-based search of the CSD with statistical analysis (e.g., histogram of O···X distances normalized by van der Waals radii) would demonstrate that the chosen examples are representative and not hand-picked outliers.
SMD Artifact for BrO₄⁻: This anomaly is a result in itself and should be analyzed, not just noted. An investigation into its physical origin could provide a valuable cautionary note for the computational community.

Author Response

Reply to reviewer 3

Recommendation: Major Revision

The Fundamental Limitation of the Model: Cluster-in-Cavity vs. True Crystal Environment

Reply: We thank the Reviewer for this important and insightful comment. We agree that periodic boundary condition (PBC) calculations would provide a more rigorous representation of the crystalline environment and could further clarify the role of long-range electrostatic and packing effects in stabilizing the observed anion–anion assemblies. The present study was not intended to quantitatively reproduce the full crystal lattice environment, but rather to examine whether the isolated XO₄⁻ anions intrinsically possess the electrophilic characteristics required for conventional halogen bonding and whether dielectric screening can qualitatively facilitate metastable anion–anion organization.

We also agree that implicit solvent models represent a simplified approximation of the condensed-phase environment and cannot fully capture the collective and cooperative interactions present in crystals. For this reason, we have explicitly clarified the limitations of the cluster-in-continuum approach in the revised manuscript and moderated several statements to avoid overinterpretation of the solvent-stabilized structures. The anomalous stabilization observed for the (BrO₄⁻)₂ system has likewise been discussed more carefully as a limitation of the continuum treatment for highly charged species.

Nevertheless, despite these limitations, the central conclusion of the work remains unchanged: the interacting halogen centers do not exhibit electrophilic σ-holes, and the observed directional contacts are therefore not consistent with conventional halogen bonding. We agree that explicit crystal-environment calculations using periodic methodologies would be valuable for future investigation of these systems.

Structural and Stylistic Issues

CSD Analysis: The selection of structures appears subjective. A systematic, script-based search of the CSD with statistical analysis (e.g., histogram of O···X distances normalized by van der Waals radii) would demonstrate that the chosen examples are representative and not hand-picked outliers.
SMD Artifact for BrO₄⁻: This anomaly is a result in itself and should be analyzed, not just noted. An investigation into its physical origin could provide a valuable cautionary note for the computational community.

Reply: We thank the Reviewer for these thoughtful comments regarding the structure and presentation of the manuscript. We agree that the original organization could obscure the central message of the work. Accordingly, Section 3 has now been substantially condensed to retain only the essential discussion necessary to illustrate that directional intermolecular organization can also arise from negative σ-holes, independent of conventional electrophilic halogen bonding. Likewise, the discussion previously presented in Section 7 has been shortened and repositioned within the manuscript to improve the logical progression of the argument and avoid interrupting the resolution of the main discussion concerning the XO₄⁻ systems.

Regarding the CSD analysis, we agree that a more systematic statistical treatment would further strengthen the presentation. The structures analyzed in the present work were selected to represent the major classes of geometrical motifs observed for the perhalate assemblies rather than isolated exceptional cases. To clarify this point, we have revised the text accordingly and included additional discussion emphasizing the representative nature of the selected structural motifs. A broader statistical analysis of the CSD geometries is currently underway and will be reported separately.

We also appreciate the Reviewer’s observation concerning the anomalous stabilization obtained for the (BrO₄⁻)₂ system within the SMD treatment. In the revised manuscript, we have expanded the discussion of this behavior and clarified that it likely reflects limitations of continuum dielectric models when applied to highly charged and weakly bound anionic assemblies. We agree that this represents an important cautionary point for the computational treatment of anti-electrostatic systems. We add that periodic treatment of these systems using VASP or Quantum Expresso would necessarily require modeling the complete ionic crystal, including the counterions and extended lattice environment. In such cases, extracting and interpreting the intrinsic interaction energy associated specifically with the anion–anion dimer becomes considerably less straightforward due to the collective contribution of long-range electrostatic and packing effects.

Reviewer 4 Report

Comments and Suggestions for Authors

The proximity of anions with one another has vexed theoretical chemists for some time. Many have tried to categorize the interactions in the language of noncovalent chemistry, such as halogen or chalcogen bonds, even though these anions repel one another without the intermediacy of other nearby species. The submitted manuscript continues in this vein, with a heavy emphasis on the positioning of extrema on the electrostatic potential surfaces. A central intent concerns whether the negative values of these maxima can be consistent with any attractions observed.

The authors are to be complimented for taking on an ambitious task, which involves interpretation of very small quantities. Many of the interaction energies are positive, i.e. repulsive, and even those that are attractive are barely so, less than 1 kcal. There are interatomic bonds that only appear upon adjustment of thresholds, and the same is true for certain extrema of the electrostatic potential. Within the NBO framework, hyperpolarization parameters are used in explanations that involve Rydberg orbitals, rather than the usual bonds, antibonds, or lone pairs that are the norm. At one point (p12) the authors abandon their usual solvation protocol when its results are not easily explainable, leading to questions about its validity at all. NCI and IGMH, which are typically different sides of the same coin, give very different qualitative views of a particular complex, and AIM bond paths seem to be questionable.

The result is that one must resort to tortured logic in elucidating the presence of any bonding, or in their quantification. In the absence of any clear bonding, the authors have found it necessary to resort to what they call “anti-electrostatic” bonds, which seem to mean a vanishingly weak attraction that is only present when other species or a polarizable continuum surround the system in question.

There are some minor issues as well. Section 5.1 discusses a phantom covalently bound ClO4- dimer which does not appear in the diagrams, which contain fairly long inter-subunit distances. To what do the authors refer here?

In summary, while it is tempting to dismiss this manuscript as making too much of very small numbers, and of providing little in the way of insights into the question of anion-anion interactions, the article contains a great deal of computational data. Perhaps this paper could be raised to the acceptance threshold by a reworking of the text to make more sense of these small quantities, while also acknowledging their limitations. It might also be worthwhile to discuss the limitations of thinking about overall electrostatic interactions in the oversimplified terms of the presence or location of surface extrema. And as a bottom line, just what are the takeaway points from this work that better explain these anion-anion interactions?

Author Response

Reply to reviewer 4

Reply: We thank the Reviewer for the careful and thoughtful assessment of the manuscript. We fully agree that the systems examined here involve extremely weak interactions that are challenging to characterize unambiguously and that their interpretation requires considerable caution. Indeed, one of the principal motivations of the present work was precisely to examine whether such directional anion–anion contacts should legitimately be interpreted within the conventional framework of halogen bonding. The present investigation was also motivated by the growing number of recent studies reporting directional anion–anion contacts in crystalline systems and interpreting them within the framework of noncovalent interaction chemistry, thereby necessitating careful examination of their physical origin and proper classification.

We do not claim that the investigated assemblies represent strongly bound dimers stabilized by intrinsic attractive interactions in the gas phase. On the contrary, our calculations consistently show that the isolated anion pairs are predominantly repulsive and become structurally accessible only in the presence of environmental stabilization, such as dielectric screening and crystal-packing effects. In this context, the term “anti-electrostatic” is not intended to imply the existence of strong classical bonding, but rather to describe the counterintuitive directional organization of like-charged species under condensed-phase conditions.

We also agree that several descriptors employed in the manuscript, including QTAIM bond paths, weak NBO donor–acceptor contributions, and reduced-density-gradient analyses, must be interpreted cautiously for diffuse and weakly interacting systems. For this reason, we deliberately relied on multiple complementary approaches rather than any single descriptor alone. The revised manuscript now clarifies these limitations more explicitly and moderates several statements where the interpretation could appear overstated.

Reply: We thank the Reviewer for pointing out this ambiguity. The covalently connected Cl₂O₈²⁻ dianion referred to in Section 5.1 corresponds specifically to the optimized structure shown in Figure 3a, in which the two ClO₄⁻ units become linked through an O–O covalent bridge during geometry optimization in solution. By contrast, the structure shown in Figure 3b remains a non-covalently associated (ClO₄⁻)₂ assembly characterized by comparatively long intermolecular O···O separations. We agree that this distinction was not sufficiently clear in the original text and figure description, and we have revised both accordingly to avoid confusion.

Reply: We thank the Reviewer for these thoughtful and constructive remarks. We agree that the interactions examined in the present work are intrinsically weak and that their interpretation requires careful discussion without overstating their physical significance. This study assists in several parts of the text to better distinguish between intrinsic gas-phase stability and environment-assisted structural organization in the condensed phase, while also explicitly acknowledging the limitations associated with interpreting extremely small interaction energies and weak topological descriptors.

We also agree that intermolecular interactions cannot be understood solely in terms of isolated extrema on electrostatic potential surfaces. Accordingly, the manuscript emphasizes more clearly that the presence or absence of σ-holes alone is insufficient to fully describe the complexity of these systems and that polarization, dielectric screening, dispersion, donor–acceptor contributions, and collective crystal-packing effects must also be considered simultaneously.

The principal takeaway of the present study is therefore not that strong intrinsic attraction exists between isolated like-charged anions, but rather that directional anion–anion organization observed in crystalline environments can emerge from a delicate balance of weak noncovalent and environmental effects without requiring conventional electrophilic halogen bonding. In this sense, the work aims to clarify the physical origin and proper interpretation of these experimentally observed structural motifs rather than to claim the existence of unusually strong anti-electrostatic bonding interactions.

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

The authors have satisfactorily responded to the concerns raised in the previous review. Therefore, I support the acceptance of this manuscript for publication.

Author Response

This reviewer already recommended acceptance of work based on his comments copied below:

Comments and Suggestions for Authors

The authors have satisfactorily responded to the concerns raised in the previous review. Therefore, I support the acceptance of this manuscript for publication.

Reviewer 3 Report

Comments and Suggestions for Authors

Rejection of Periodic Boundary Condition (PBC) Calculations

The authors’ response fails to address the fundamental methodological challenge raised by the reviewer. The central conclusion - that crystal packing forces, not intrinsic halogen bonding, stabilize anion–anion contacts - cannot be supported by cluster-in-continuum SMD calculations alone. The anomalous behavior of (BrO₄⁻)₂ further weakens confidence in the model.

To make the manuscript acceptable, the authors must perform periodic boundary condition DFT calculations on at least one or two representative crystal structures to directly demonstrate the role of the environment.

This author’s response on this question is unacceptable. The reviewer did not ask for a “quantitative reproduction” of the crystal lattice but rather for a qualitatively appropriate model to test the claim that the environment enables the interaction. The SMD model cannot distinguish between genuine crystal packing forces and an unstructured dielectric continuum. Agreeing that PBC would be better while declining to perform it - and then retaining the original conclusion - renders the central argument unsubstantiated. Adding cautionary language does not fix the methodological flaw. Until PBC calculations are performed, the manuscript’s core thesis remains unproven.

Without these substantive changes, the manuscript’s conclusions are not adequately supported by the presented computational methodology, and the paper should not be published in its current form.

Major revision.

Author Response

Reviewer 3

Rejection of Periodic Boundary Condition (PBC) Calculations

Without these substantive changes, the manuscript’s conclusions are not adequately supported by the presented computational methodology, and the paper should not be published in its current form.

Major revision.

Response:
We thank the reviewer for this important comment and agree that explicit periodic boundary condition (PBC) calculations are necessary to properly evaluate the role of the crystal environment in stabilizing the observed short anion–anion contacts. In response, we have replaced the previous continuum-based discussion with periodic DFT-PBE calculations on experimentally derived crystal structures.

Periodic geometry optimizations were performed for the perchlorate-containing system [Sn(H₂O)₃]²⁺·2(ClO₄⁻) using both fixed-cell (ISIF = 2) and fully relaxed (ISIF = 3) conditions. In both cases, the experimentally observed crystal packing and short ClO₄⁻···ClO₄⁻ contacts are preserved. Full relaxation results in only a minor energetic change (~0.040 eV), indicating that the experimental structure corresponds to a stable minimum within the periodic crystal environment.

For comparison, the corresponding bromate-substituted system [Sn(H₂O)₃]²⁺·2(BrO₄⁻) was optimized under identical periodic conditions. In contrast to the perchlorate case, this system exhibits a substantially larger relaxation energy (~0.512 eV), together with lattice expansion and symmetry lowering, indicating a significantly greater structural response to periodic crystal constraints.

We believe that our periodic results address the reviewer’s concern by explicitly incorporating the crystal environment. The contrasting behavior between the two systems is consistent with a strong influence of the periodic packing field, as evidenced by the stability of the perchlorate structure under full relaxation and the pronounced structural reorganization observed for the bromate analogue. These findings support the conclusion that the geometry and stability of the observed anion–anion contacts are governed by crystal packing effects in the solid state.

Reviewer 4 Report

Comments and Suggestions for Authors

While the replies to the original comments in the reviewer reply note seem reasonable, it is not clear how the manuscript has been changed to include them. The only red text comprises a new Section 4 that is unrelated to the issues raised earlier.

Round 3

Reviewer 3 Report

Comments and Suggestions for Authors

Accept in present form