**4. Conclusions**

To summarize, the paper showed an application of quantum-mechanical calculations for the detection of particular physical-chemical processes in a molecule. DFT calculations enabled the prediction of the possible observation of two phenomena for the studied compound: polymorphism in the solid state and the conformational equilibrium under the condition approaching to the gas phase. The first of the predicted phenomena—the existence of two polymorphs of the studied compound and the phase transition between them-was revealed by DSC and NQR techniques. The non-equivalency of the structures in crystal cells was reflected in the splitting of the bands in the IINS spectrum, and the bands in IR spectrum registered in far-infrared region.

The second of the predicted phenomena was the conformational equilibrium between two hydrogen bonds (N = O··· H–O and O–H··· O = C(CH3)). Two conformational forms detected in the matrix isolation conditions by infrared spectroscopy unambiguously proved this prediction. Complete assignments of spectral bands based on the PED analysis and isotopic substitution showed the presence of two conformers under the matrix condition and two polymorph forms in the solid state. The studies confirmed the agreemen<sup>t</sup> of the theoretical and experimental results.

**Supplementary Materials:** The following are available online, Figures S1–S8, Table S1–S5.

**Author Contributions:** Conceptualization, A.F.; methodology, Ł.H., P.S., A.K. and M.P.-K.; software, Ł.H., P.S., M.P.-K. and A.K.; validation, A.F.; formal analysis, all Authors; investigation, Ł.H., P.S., A.K., M.W., M.Y.M., P.M.T., M.P.-K. and A.F.; resources, P.M.T. and A.F.; data curation, Ł.H., P.S., A.K., M.W., M.Y.M., P.M.T., M.P.-K. and A.F.; writing—original draft preparation, Ł.H., P.S., A.K., M.W., M.Y.M., P.M.T., M.P.-K. and A.F.; writing—review and editing, A.F.; visualization, Ł.H., P.S., A.K., M.W., M.Y.M., P.M.T., M.P.-K. and A.F.; supervision, A.F.; project administration, A.F.; funding acquisition, M.Y.M., P.M.T. and A.F. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by RSF, gran<sup>t</sup> number no: 18-13-00050.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** All data associated with this article are included in Supplementary Materials.

**Acknowledgments:** This work was supported by RSF (no: 18-13-00050) grant. The Authors acknowledge the Wrocław Centre for Networking and Supercomputing Centres (WCSS) for providing computational time and facilities.

**Conflicts of Interest:** The authors declare no conflict of interest.

**Sample Availability:** Samples of the compound 5-chloro-3-nitro-2-hydroxyacetophenone are available from the authors.
