**4. Conclusions**

The paper presents the synthesis of 2-[(E)-(phenylimino)methyl]phenol (**SA**) and its two deutero-derivatives (**SA-OD** and **SA-C6D5**). The bands corresponding to hydrogen bond vibrational modes (see Scheme 1) were assigned in experimental IR, IINS and Raman spectra. The analysis of the obtained spectral results proved that in a crystal state, **SA** exists in enol-imine form with a so-called quasi-aromatic intramolecular OH···N hydrogen bond of medium strength. The measured IINS spectra **SA** and **SA-C6D5** made it possible to show the influence of deuteron replacement in the remote group (aldimine ring) on the hydrogen bridge stretching vibrations <sup>ν</sup>σ(OHN). The calculations of structure and vibrational spectra of two polymorphs (earlier studied by X-ray method [27]) performed by static periodic (solid state) DFT method showed a good agreemen<sup>t</sup> between the measured and calculated spectra, allowing for spectroscopic distinction between polymorphs α2 and β, as well as confirming the applicability of DFPT calculations for characterization of polymorphic states. Based on analysis of IR spectra, the **SA** in the studied samples was shown to exist in α-form. However, the XPD measurements carried out for **SA** and **SA-C6D5** showed dual signals

for several reflexes and did not show a phase transition in the temperature range from 20 to 295 K. On the basis of these measurements two quasi-isostructural polymorphs of **SA** (most probably, so-called α1 and α2 forms) were assumed to co-exist in the mentioned temperature range.

**Supplementary Materials:** The following are available online, Figure S1: Normalized experimental Raman and IR spectra of **SA** and its deuterated derivative **SA-OD**. Figure S2: Temperature behaviour of XRD patterns for **SA** and **SA-C6D5**. Figure S3: Temperature dependence of X-ray powder diffraction patterns of **SA** and **SA-C6D5.** Table S1: Definitions of the internal coordinates used in the potential energy distribution (PED) analysis for the assignments of the vibrational spectra, Table S2: Experimental IR, Raman, IINS and calculated (B3LYP/6-311+G(d,p)) spectral data of **SA**, Table S3: Experimental IR, Raman and calculated (B3LYP/6-311+G(d,p)) spectral data of **SA-OD**, Table S4: Experimental IINS and calculated (B3LYP/6-311+G(d,p)) spectral data of **SA-C6D5** for the region 0–1200 cm–1.

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

**Funding:** This research was funded by the Russian Science Foundation (RFBR, gran<sup>t</sup> no 18-13-00050) and Polish Government Plenipotentiary for JINR in Dubna (PWB/168-19/2021; data 11.03.2021).

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

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** All data are available within the article or Supplementary Materials.

**Acknowledgments:** A.F. acknowledges 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 studied compounds are not available from the authors.
