**2. Results and Discussion**

#### *2.1. Assignment of Hydrogen Bonding Vibrations*

This part of the paper focuses on the assignments of the spectral bands to the vibrations involving the hydroxyl group (ν(OH), δ(OH) and γ(OH))) and the hydrogen bridge (<sup>ν</sup>σ(ON) and <sup>ν</sup>β(ON)), Scheme 1, continuing and expanding our studies started in Ref. [38].

**Scheme 1.** The OHN hydrogen bond vibrations of the studied 2-[(E)-(phenylimino)methyl]phenols.

We start the discussion with the high-frequency region of vibrational spectra, which are presented in Figure S1 in the supporting information for brevity. The broad band located within the range of 2900–1500 cm<sup>−</sup><sup>1</sup> in the measured infrared spectrum is assigned to the stretching vibrations ν(OH) as a result of the comparison of the spectra of the nondeuterated and deuterated isopologues (ISR = 1.320). This band is characteristic for OH form of Schiff bases (enol-imine) and not to NH form (keto-amine). The ν(OH) band is rather wide and can be assigned to the so-called Zundel's continuum absorption [39]. This fact proves that the hydrogen bond in these compounds is strong. Moreover, the intensity of the ν(OH) band is rather weak despite its noticeable shift towards lower frequencies, which indicates that the hydrogen bond in **SA** is a quasi-aromatic one, which is also referred to as resonance-assisted hydrogen bond (RAHB) [40–48].

The low-frequency region of the IR, Raman and IINS spectra of non-deuterated (**SA**) and deuterated (**SA-OD**) compounds are shown in Figure 3. The experimental spectra were interpreted using the calculated vibrational spectra (DFT) and the results of the PED analysis (Tables S2–S4).

**Figure 3.** Normalized experimental IINS ( **A**), Raman (**B**) and IR ( **C**) spectra of **SA** (black line) and its deuterated derivative **SA-OD** (red lines).

When it comes to the in-plane deformational vibrations δ(OH), they are not characteristic for ortho-hydroxy Schiff bases [33,35,38]. According to PED analysis, δ(OH) vibration is mixed with the ν(Calk Calk) and γ(CH) vibrations. After the OH → OD substitution the bands located at 1571 and 1484 cm<sup>−</sup><sup>1</sup> (both IR and Raman active) decrease in intensity and shift slightly, while the more characteristic δ(OD) bands appear at ca. 1130 cm<sup>−</sup><sup>1</sup> and ca. 1020 cm<sup>−</sup><sup>1</sup> both in IR and Raman spectra of **SA-OD** (Figure 3).

As for the out-of-plane deformational vibration γ(OH), in the IR spectrum of **SA** the corresponding band is found in the range of 860–820 cm<sup>−</sup>1, but it strongly overlaps with the bands of other vibrations and was identified with the help of DFT calculations. However, in the IR spectra of **SA-OD** this band is clearly visible at 601 cm<sup>−</sup>1. The γ(OH) vibration of **SA** is practically Raman-inactive, as in the 860–820 cm<sup>−</sup><sup>1</sup> region of the Raman spectrum there are no clearly identifiable bands which would be sensitive to the OH → OD replacement. The band at 848 cm<sup>−</sup><sup>1</sup> in the IINS spectrum drops in intensity after the deuteration and thus it was assigned to the γ(OH) vibration. Note, that for IINS method the scattering cross-section for the deutron is much smaller than that of the proton and thus the contribution of the vibrations involving deutrons in IINS spectra is weak. The experimentally observed positions of γ(OH) bands and the H/D isotope effects on them are consistent with the literature data [49–51].

The stretching vibration of the hydrogen bridge (<sup>ν</sup>σ(OHN)) gives rise to the isotopesensitive bands at 449 cm<sup>−</sup><sup>1</sup> in the IINS spectrum and two bands at 448 and 434 cm<sup>−</sup><sup>1</sup> in the Raman spectrum of **SA** (Figure 3). The intensity of these bands is greatly decreased in the IINS and the Raman spectra of **SA-OD**, while a new band appears at 425 cm<sup>−</sup><sup>1</sup> in the Raman spectrum, assigned to the vibration of the ODN bridge, <sup>ν</sup>σ(ODN). The

IINS spectrum does not show the band of the deformational vibration <sup>ν</sup>β(OHN) due to insignificant deformational motion of the bridged proton. This phenomenon was described earlier in Ref. [38]. To make the interpretation of the hydrogen bridge vibrations more accurate and reliable, the synthesis of **SA-C6D5** isotopologue was performed (deuteration in the aldimine ring, NC6H5 → NC6D5). In Figure 4 the IINS spectra of **SA**, **SA-OD** and **SA-C6D5** are compared. Upon deuteration in the hydrogen bridge site (**SA-OD**), the intensity of the bands assigned to the γ(OH) and <sup>ν</sup>σ(OHN) vibrations (848 cm<sup>−</sup><sup>1</sup> and 449 cm<sup>−</sup>1, respectively) is decreasing (Figure 4A), while the IINS spectrum of the **SA-C6D5** derivative features a more complicated picture (Figure 4B). Firstly, the IINS spectrum of **SA-C6D5** is characterized by the disappearance of a number of bands assigned to γ(CH) and τ(CH) vibrations, which were located at 1183, 1175, 1153, 1089, 703 and 521 cm<sup>−</sup><sup>1</sup> in the spectrum of **SA** (see blue arrows in Figure 4). Secondly, the bands visible at 208, 264, 359 and 495 cm<sup>−</sup><sup>1</sup> in the spectrum of **SA** (see red arrows in Figure 4B) shift to lower frequencies, namely, to 202, 255, 345 and 491 cm<sup>−</sup>1, respectively. The intensity of the first three bands is decreasing, while that of the fourth one is increasing slightly. Judging from Tables S2 and S4, these bands stem from the skeleton vibrations of the aldimine ring, where the deuteration occurs.

**Figure 4.** Normalized IINS spectra of **SA** (black traces), **SA-OD** (panel (**A**), red trace) and **SA-C6D5** (panel (**B**), blue trace). Blue arrows: bands of γ(CH) and τ(CH) vibrations, which disappear after deuteration. Red arrows: bands of various vibrations, which shift after the deuteration. See text for more details.

The band at 449 cm<sup>−</sup>1, which is assigned to <sup>ν</sup>σ(OHN) vibration (also marked by the red arrow in Figure 4B), is also sensitive to deuteration in the aldimine ring: the band broadens and its peak intensity decreases. Such a long-range H/D isotope effect on hydrogen bond vibrations is reported here for the first time. Below in Section 2.3 it is shown that **SA** exists as a mixture of two quasi-isostructural polymorphs and the shift of the <sup>ν</sup>σ(OHN) band might be associated with the change of the mole fractions of these polymorphs upon **SA** → **SA-C6D5** deuteration, though this question requires additional consideration which is beyond the scope of this work.

## *2.2. Spectral Manifestations of Polymorphism*

The **SA** could crystallize in three polymorphic states—called α1, α2 and β—the crystal structures and cell packing of which were earlier published by F. Arod in papers [27,28]; for a visual representation of the cell packing see Figure 2 (polymorph β) and Figure 8 (polymorphs α1 and α2) in [27]. All three polymorphs exhibit enol-imine form with intramolecular OH···N hydrogen bond. These polymorphs differ only slightly in molecular positions and conformations, representing "very close points in the crystal structure landscape" [52–54]; one of the larger differences between α and β states is the rotational conformation of the aldimine ring (Chart **c**, Figure 1). The polymorphs α1 and α2 are called quasi-isostructural. The structural mobility and polymorphism of different compounds [55–59], and Schiff bases in that number [60,61], was investigated by various methods. In this part of the paper our goal was to determine if vibrational spectra could be used to unambiguously identify the particular polymorphic state and which vibrational marker modes would be most informative. For that purpose, the crystallographic structures of polymorphs α2 and β were optimized with CRYSTAL software, the IR spectra were calculated by DFPT method and compared with the experimental one. The result of the comparison is shown in Figure 5. There is a reasonably good agreement—both in bands positions and their relative intensities—between the experimental spectrum (Figure 5A) obtained at 295 K in which polymorph α2 prevails and the calculated spectrum of polymorph α2 (Figure 5B), while the agreemen<sup>t</sup> with that of polymorph β is significantly worse (Figure 5C). The most informative bands are marked by asterisks in Figure 5A,B. These bands are assigned to the following vibrations: ν(C=N), ν(Car Car), γ(CH), τ(CH) and τ(CC).

**Figure 5.** Normalized IR experimental (( **A**), black trace; measured at 295 K) and calculated by DFPT method spectra of polymorphs α2 ((**B**), blue trace) and β ((**C**), green trace) of **SA**. Black and blue asterisks: the most informative bands, allowing one to identify polymorph α2. See text for more details.

The abovementioned observations support the applicability of DFPT computational method for the research of polymorphic states.

#### *2.3. X-ray Powder Diffraction (XPD) Study of Polymorphism in SA*

X-ray Powder Diffraction measurements of **SA** and **SA-C6D5** were carried out in the 20–320 K temperature range. The X-ray diffraction pattern for **SA-OD** is not discussed here, because the results closely match those for **SA**. The experiments revealed that both **SA** and **SA-C6D5** crystallize in a triclinic form, which is in agreemen<sup>t</sup> with the single crystal X-ray data for polymorph α1 obtained earlier [27]. For **SA**, several reflexes are observed as dual signals in the 20–295 K temperature range. As an example, in Figure 6 the reflexes, 002 and 0-11, for **SA** and **SA-C6D5** are shown. The positions and relative intensities of components of the dual signals are temperature dependent. Similar observations are valid for the deuterated derivative **SA-C6D5** (Figure 6C,D). Such behaviour is often attributed to the co-existence of two quasi-isostructural polymorphs, preserving the same crystal symmetry [52–56]. In case of **SA**, following the results of Refs. [27,28] we assign these polymorphs to α1 and α2 forms.

**Figure 6.** Temperature behaviour of (002) (**A**,**C**) and (0-11) (**B**,**D**) reflections of **SA** top (**A**,**B**) and **SA-C6D5**bottom (**C**,**D**) panels.

For a better understanding of this phenomenon we performed DFT calculations (in the gas phase) of the potential curves for the rotation of the aldimine fragment of **SA** in the enol-imine and keto-amine forms. The calculation confirmed that the structure of the keto-amine form is evidently flat and the twist of the aldimine fragment by up to 20◦ virtually does not change the potential energy (Figure 7, top). In contrast, the optimized geometry of the enol-imine form is not flat: torsional angle Θ(C=N-C=C) = 40◦ (Figure 7, bottom) and crossing of the phenol ring plane requires to overcome a 1 kcal/mol barrier. According to the postulate of Benstein et al. [62], such a barrier in a non-homogeneous environment of a crystal lattice could make it possible to obtain two polymorphic forms.

Though the keto-amine form is less stable than the enol-imine form by ca. 4.6 kcal/mol and unlikely to be present at room temperature, one could speculate see that the flat structure of the keto-amine form would not be prone to polymorphism.

The keto-amine if the keto-amine form would be,

**Figure 7.** The calculated potential energy profile (B3LYP/6-311++G(d,p)) for a gradual rotation around the N-C bond for enol-imine (blue line) and keto-amine (green line) forms of studied Schiff base.

Upon further heating, a significant change of diffractograms of **SA** and **SA-C6D5** is observed at ca. 310 K (see the set of diffractograms in Figures S2 and S3). Based on the available XPD data, it is difficult to speculate which structural changes are responsible for this, but it is unlikely to be the α ↔ β transition, because the melting temperature of studied crystal was 325 K, coinciding with that previously reported for α1 in Ref. [27].

## **3. Materials and Methods**

*3.1. Compounds and Deuteration*

2-[(E)-(phenylimino)methyl]phenol (**SA**) and 2-[(E)-(phenyl-D5-imino)methyl]phenol (**SA-C6D5**) were synthesized from stoichiometric mixtures of the salicylaldehyde with aniline or aniline-D5 in refluxing methanol, respectively. The solvents were purchased from *Sigma-Aldrich* and used without further purification. For the deuteration in the mobile proton site, the solution of 2-[(E)-(phenylimino)methyl]phenol in methanol-OD was heated to 60 ◦C and refluxed during 30 min, then the methanol was removed by evaporation, leaving **SA-OD**. The deuteration degree was estimated to be ca. 90%.

#### *3.2. Infrared, Raman and IINS Measurements*

The infrared measurements were performed using a *Bruker Vertex 70v* spectrometer. The spectra were collected with a resolution of 2 cm<sup>−</sup>1. The FT-FIR spectra (500–50 cm<sup>−</sup>1) were collected for sample suspended in Apiezon N grease and placed on a polyethylene (PE) disc. The FT-MIR spectra were collected for sample in a KBr pellet. The Raman spectra of the analysed samples were obtained using an FT-Nicolet Magma 860 spectrophotometer. The In:Ga:Ar laser excitation at 1064 nm was employed for the Raman measurements. The spectra were recorded at the room temperature with the spectral resolution 4 cm<sup>−</sup><sup>1</sup> and with 512 scans. Neutron scattering data were collected at the pulsed IBR-2 reactor in the Joint Institute of Nuclear Research (Dubna, Russia) using the time-of-flight inverted geometry spectrometer NERA at 10 K temperature. The experimental features are described in Ref. [50].
