Review Reports

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Manuscript ID inorganics-3975609 presents a thorough structural and computational analysis of the potassium salt of protocatechuic acid (PCA), a humic substance of environmental relevance. The X-ray data collection evidenced a complex structure where there is a struggle between the catecholic and carboxylic functions to coordinate the potassium ion.

This work makes a valuable contribution to the fields of solid-state chemistry and computational materials science, particularly with regard to understanding the complex coordination environments of ions in organic salts. I recommend publication following minor revisions to clarify certain methodological aspects and strengthen the discussion. The required corrections are listed below.

• Title: correct Cathecholic in Catecholic.
• The authors stated: ‘This .cif is partially incomplete – .i.e. misses s.u. on hydrogen atoms. This is due to the fact that this is an old data collection (2016) and the file bearing all the relative information was lost.’ Why didn't the authors try to collect a new set of data?
• The MD simulation time was 7 ps, which is notably shorter than the 1 ns force-field runs. While acknowledging the computational cost, the authors should briefly comment on whether this timeframe was sufficient for convergence of the studied properties.
• Table 1: Add a column showing the percentage deviation of each computed cell parameter from the experimental value.

Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

This paper reports on X-ray diffraction structure determination of potassium 3,4-dihydroxybenzoate and computational study of its crystal. It is generally concise and interesting to read. There are some minor points listed below.

Reference selection for introduction and discussion create the impression of excessive self-citation to some extent. Recent studies on structures of 4-hydroxybenzates and 2,4,6-trihydroxybenzates [10.1107/S2053229622009901][10.1107/S2053229621005465] are probably closely enough related for introduction.

Discussion of thermal motion relation and T-shaped π...HC interaction (lines 163-189 including figure 6) is difficult to comprehend. It is easy to assume that π...HC contact is the least directed (and probably the weakest) of the interactions that form the crystal lattice of compound under study resulting in larger thermal motion of participating atoms. Even if this simple hypothesis is wrong according to computational data, its probably good start for the discussion.

PCA abbreveation introduced for 3,4-dihydroxybenzoic acid at the begining is also used for potassium salt under study throughout the manuscript which is misleading.

Experimental part lacks the description of crystal preparation.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The authors have corrected most of the issues raised by the reviewer, and the paper is now in a form that is acceptable for publication.