Spectroscopic Identification of Hydrogen Bond Vibrations and Quasi-Isostructural Polymorphism in N-Salicylideneaniline

The ortho-hydroxy aryl Schiff base 2-[(E)-(phenylimino)methyl]phenol and its deutero-derivative have been studied by the inelastic incoherent neutron scattering (IINS), infrared (IR) and Raman experimental methods, as well as by Density Functional Theory (DFT) and Density-Functional Perturbation Theory (DFPT) simulations. The assignments of vibrational modes within the 3500–50 cm−1 spectral region made it possible to state that the strong hydrogen bond in the studied compound can be classified as the so-called quasi-aromatic bond. The isotopic substitution supplemented by the results of DFT calculations allowed us to identify vibrational bands associated with all five major hydrogen bond vibrations. Quasi-isostructural polymorphism of 2-[(E)-(phenylimino)methyl]phenol (SA) and 2-[(E)-(phenyl-D5-imino)methyl]phenol (SA-C6D5) has been studied by powder X-ray diffraction in the 20–320 K temperature range.


Introduction
This paper dwells on the studies of one of the most popular photo-thermochromic compounds, 2-[(E)-(phenylimino)methyl]phenol (N-salicylideneaniline), from the group of the ortho-hydroxy aryl Schiff bases. The first compounds of this type were synthesized in 1864 by Hugo Schiff [1] and have attracted attention ever since [2,3]. The ortho-hydroxy aryl Schiff bases, and materials based on them, demonstrate a number of interesting and useful characteristics. For example, they manifest polymorphic properties [4], ionic liquids' properties [5], elastic bending capability [6] and recognised anti-cancer properties [7]. The chiral aldimines possess the photochromic feature in the crystalline state, caused by photoinduced proton transfer in the intramolecular OHN hydrogen bond [8,9]. The chiroptical and optical anisotropic properties of photomechanical Schiff bases were previously studied under UV irradiation [10]. Such properties as ferroelectricity, piezoelectricity and second-order optical non-linearity were reported [11,12].
Many of the abovementioned features are linked to the existence of an intramolecular OHN hydrogen bond and to various conformational changes that ortho-hydroxy aryl Schiff The recent comprehensive studies seem to indicate that the proton transfer in the intramolecular OHN hydrogen bond and the isomerization of the aldimine fragment (Chart b, Figure 1) is in charge of the appearance of photo-thermochromic properties in ortho-hydroxy aryl Schiff bases [28]. Notably, the strength of the hydrogen bond and tautomeric equilibrium in it are linked to the observation of certain physical-chemical properties. In cases where the OH tautomeric form is prevailing, it is possible to observe OH isomerization, leading to the absence of photo-thermochromic properties, Chart a. Recently, Mielke et al. [29,30] have discovered the existence of the trans-OH form in the specific condition of matrix isolation. However, the prevailing of the NH tautomeric form makes it possible to observe the isomerization of the amino group (Chart b) and, therefore, the emergence of photo-thermochromic properties. It is important to underline the contribution by Ogawa et al. [24] who were the first to show the existence of trans-NH form in the solid state by X-ray method. The possibility of rotation of aldimine ring (Chart c) was first demonstrated by Cohen et al. [13,20], and later it was associated with the appearance of polymorphism of the studied compound.
The conformational changes, occurring in ortho-hydroxy aryl Schiff bases in different conditions and states, were investigated mostly by the methods of vibrational spectroscopy in the middle spectral range [29][30][31][32][33][34][35][36], while the low-and high-frequency hydrogen bridge vibrations have never been in the focus of the spectroscopic studies before. Therefore, in order to fill in the gap, we have synthesized 2-[(E)-(phenylimino)methyl]phenol (N-salicylideneaniline, SA) and its deutero-derivatives (SA-OD and SA-C6D5, see Figure  2) to analyse the vibrational spectra measured by IINS, Raman and IR methods. The quantum-mechanical DFT and DFPT calculations, as well as Potential Energy Distribution (PED) analysis, have been conducted to assign the spectral bands. The isotopic replacements of the bridged hydrogen for deuterium (O-H···N → O-D···N) and the hydrogen atoms for the deuterium atoms in aldimine ring (NC6H5 → NC6D5) were performed to additionally confirm the assignment. To that end, the Isotopic Spectral Ratios (ISRs) were used, which are defined for a given vibrational mode as the ratio of frequencies for nondeuterated and deuterated species. The ISR values are well studied for high-frequency The recent comprehensive studies seem to indicate that the proton transfer in the intramolecular OHN hydrogen bond and the isomerization of the aldimine fragment (Chart b, Figure 1) is in charge of the appearance of photo-thermochromic properties in ortho-hydroxy aryl Schiff bases [28]. Notably, the strength of the hydrogen bond and tautomeric equilibrium in it are linked to the observation of certain physical-chemical properties. In cases where the OH tautomeric form is prevailing, it is possible to observe OH isomerization, leading to the absence of photo-thermochromic properties, Chart a. Recently, Mielke et al. [29,30] have discovered the existence of the trans-OH form in the specific condition of matrix isolation. However, the prevailing of the NH tautomeric form makes it possible to observe the isomerization of the amino group (Chart b) and, therefore, the emergence of photo-thermochromic properties. It is important to underline the contribution by Ogawa et al. [24] who were the first to show the existence of trans-NH form in the solid state by X-ray method. The possibility of rotation of aldimine ring (Chart c) was first demonstrated by Cohen et al. [13,20], and later it was associated with the appearance of polymorphism of the studied compound.
The conformational changes, occurring in ortho-hydroxy aryl Schiff bases in different conditions and states, were investigated mostly by the methods of vibrational spectroscopy in the middle spectral range [29][30][31][32][33][34][35][36], while the low-and high-frequency hydrogen bridge vibrations have never been in the focus of the spectroscopic studies before. Therefore, in order to fill in the gap, we have synthesized 2-[(E)-(phenylimino)methyl]phenol (Nsalicylideneaniline, SA) and its deutero-derivatives (SA-OD and SA-C 6 D 5 , see Figure 2) to analyse the vibrational spectra measured by IINS, Raman and IR methods. The quantummechanical DFT and DFPT calculations, as well as Potential Energy Distribution (PED) analysis, have been conducted to assign the spectral bands. The isotopic replacements of the bridged hydrogen for deuterium (O-H···N → O-D···N) and the hydrogen atoms for the deuterium atoms in aldimine ring (NC 6 H 5 → NC 6 D 5 ) were performed to additionally confirm the assignment. To that end, the Isotopic Spectral Ratios (ISRs) were used, which are defined for a given vibrational mode as the ratio of frequencies for non-deuterated and deuterated species. The ISR values are well studied for high-frequency stretching and bending proton vibrations [37], but poorly studied for the low-frequency hydrogen bridge vibrations. Moreover, this paper delves into the spectroscopic manifestations of polymorphism of N-salicylideneaniline and its isotopologues in a wider temperature range in 20-320 K. N-salicylideneaniline was studied earlier in papers [27,28] in the 100-320 K temperature range.

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 (νσ(ON) and νβ(ON)), Scheme 1, continuing and expanding our studies started in Ref. [38]. 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 −1 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][41][42][43][44][45][46][47][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). Moreover, this paper delves into the spectroscopic manifestations of polymorphism of N-salicylideneaniline and its isotopologues in a wider temperature range in 20-320 K. N-salicylideneaniline was studied earlier in papers [27,28] in the 100-320 K temperature range.

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 (ν σ (ON) and ν β (ON)), Scheme 1, continuing and expanding our studies started in Ref. [38]. Moreover, this paper delves into the spectroscopic manifestations of polymorphism of N-salicylideneaniline and its isotopologues in a wider temperature range in 20-320 K. N-salicylideneaniline was studied earlier in papers [27,28] in the 100-320 K temperature range.

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 (νσ(ON) and νβ(ON)), Scheme 1, continuing and expanding our studies started in Ref. [38]. 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 −1 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][41][42][43][44][45][46][47][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). 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 −1 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][41][42][43][44][45][46][47][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  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(CalkCalk) and γ(CH) vibrations. After the OH → OD substitution the bands located at 1571 and 1484 cm −1 (both IR and Raman active) decrease in intensity and shift slightly, while the more characteristic δ(OD) bands appear at ca. 1130 cm −1 and ca. 1020 cm −1 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 −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 −1 . The γ(OH) vibration of SA is practically Raman-inactive, as in the 860-820 cm −1 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 −1 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][50][51].
The stretching vibration of the hydrogen bridge (νσ(OHN)) gives rise to the isotopesensitive bands at 449 cm −1 in the IINS spectrum and two bands at 448 and 434 cm −1 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 −1 in the Raman spectrum, assigned to the vibration of the ODN bridge, νσ(ODN). The IINS spectrum 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(C alk C alk ) and γ(CH) vibrations. After the OH → OD substitution the bands located at 1571 and 1484 cm −1 (both IR and Raman active) decrease in intensity and shift slightly, while the more characteristic δ(OD) bands appear at ca. 1130 cm −1 and ca. 1020 cm −1 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 −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 −1 . The γ(OH) vibration of SA is practically Raman-inactive, as in the 860-820 cm −1 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 −1 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][50][51].
The stretching vibration of the hydrogen bridge (ν σ (OHN)) gives rise to the isotopesensitive bands at 449 cm −1 in the IINS spectrum and two bands at 448 and 434 cm −1 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 −1 in the Raman spectrum, assigned to the vibration of the ODN bridge, ν σ (ODN). The IINS spectrum does not show the band of the deformational vibration ν 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-C 6 D 5 isotopologue was performed (deuteration in the aldimine ring, NC 6 H 5 → NC 6 D 5 ). In Figure 4 the IINS spectra of SA, SA-OD and SA-C 6 D 5 are compared. Upon deuteration in the hydrogen bridge site (SA-OD), the intensity of the bands assigned to the γ(OH) and ν σ (OHN) vibrations (848 cm −1 and 449 cm −1 , respectively) is decreasing ( Figure 4A), while the IINS spectrum of the SA-C 6 D 5 derivative features a more complicated picture ( Figure 4B). Firstly, the IINS spectrum of SA-C 6 D 5 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 −1 in the spectrum of SA (see blue arrows in Figure 4). Secondly, the bands visible at 208, 264, 359 and 495 cm −1 in the spectrum of SA (see red arrows in Figure 4B) shift to lower frequencies, namely, to 202, 255, 345 and 491 cm −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.
formational 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 νσ(OHN) vibrations (848 cm −1 and 449 cm −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 −1 in the spectrum of SA (see blue arrows in Figure 4). Secondly, the bands visible at 208, 264, 359 and 495 cm −1 in the spectrum of SA (see red arrows in Figure 4B) shift to lower frequencies, namely, to 202, 255, 345 and 491 cm −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.
The band at 449 cm −1 , which is assigned to νσ(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 νσ(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.  The band at 449 cm −1 , which is assigned to ν 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 ν σ (OHN) band might be associated with the change of the mole fractions of these polymorphs upon SA → SA-C 6 D 5 deuteration, though this question requires additional consideration which is beyond the scope of this work.

Spectral Manifestations of Polymorphism
The SA could crystallize in three polymorphic states-called α 1 , α 2 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 α 1 and α 2 ) 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][53][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 α 1 and α 2 are called quasi-isostructural. The structural mobility and polymorphism of different compounds [55][56][57][58][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 α 2 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 α 2 prevails and the calculated spectrum of polymorph α 2 ( Figure 5B), while the agreement 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(C ar C ar ), γ(CH), τ(CH) and τ(CC).

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][53][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][56][57][58][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 agreement 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(CarCar), γ(CH), τ(CH) and τ(CC).
The abovementioned observations support the applicability of DFPT computational method for the research of polymorphic states.   The abovementioned observations support the applicability of DFPT computational method for the research of polymorphic states.

X-ray Powder Diffraction (XPD) Study of Polymorphism in SA
X-ray Powder Diffraction measurements of SA and SA-C 6 D 5 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-C 6 D 5 crystallize in a triclinic form, which is in agreement with the single crystal X-ray data for polymorph α 1 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-C 6 D 5 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-C 6 D 5 (Figure 6C,D). Such behaviour is often attributed to the co-existence of two quasi-isostructural polymorphs, preserving the same crystal symmetry [52][53][54][55][56]. In case of SA, following the results of Refs. [27,28] we assign these polymorphs to α 1 and α 2 forms.
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 agreement 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][53][54][55][56]. In case of SA, following the results of Refs. [27,28] we assign these polymorphs to α1 and α2 forms.
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,  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, 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].

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 −1 . The FT-FIR spectra (500-50 cm −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 −1 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]. Upon further heating, a significant change of diffractograms of SA and SA-C 6 D 5 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 α 1 in Ref. [27].

Compounds and Deuteration
2-[(E)-(phenylimino)methyl]phenol (SA) and 2-[(E)-(phenyl-D 5 -imino)methyl]phenol (SA-C 6 D 5 ) were synthesized from stoichiometric mixtures of the salicylaldehyde with aniline or aniline-D 5 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%.

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 −1 . The FT-FIR spectra (500-50 cm −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 −1 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].

X-ray Powder Diffraction
The powder X-ray measurements were performed using EMPYREAN equipped with the water-cooled X-ray tube with Cu anode with the wavelengths of characteristic radiation λ CuKα1 = 1.54056 Å and λ CuKα2 = 1.54443 Å. In order to suppress the K β radiation the Ni filter was used. The central part of the data acquisition system was the silicon strip detector PIXcel. The low temperature measurements were made using the closed cycle helium cryostat Phenix-Oxford Cryosystems.

Calculations
Quantum-mechanical calculations using the B3LYP functional (DFT, the three-parameter exchange hybrid functional of Becke [63], and gradient-corrected correlation functional of Lee, Yang and Parr) [64] with the 6-311++G(d,p) basis set [65][66][67] were performed for full geometry optimizations with the Gaussian 16 Rev. C01 suite of program [68]. The structures were visualized by the MOLDEN program [69]. The potential energy distribution (PED) of the normal modes was calculated in terms of natural internal coordinates using the Gar2ped program [70]. Static periodic (solid-state) DFT calculations (DFPT Density-Functional Perturbation Theory) were performed in the CRYSTAL09 [71,72] software package using the B3LYP functional with the Grimme D2 dispersion correction [73] and 6-31G** basis set [67]. The space groups and unit cell parameters of the considered crystals obtained in the experimental studies [27,28] were fixed and structural relaxations were limited to the position of atoms. Such approximation gives a reasonable description of the structure and spectroscopic features of intra-and intermolecular H-bonds of different types and strengths in molecular crystals [74][75][76]. The experimental atomic positions were used as the starting point in the periodic DFT computations. Periodic DFT computations were conducted for polymorphs α 2 and β. The disordered α 1 -polymorph [27] is not suitable for periodic calculations. Since α 1 and α 2 polymorphs are quasi-isostructural, we assumed that their IR spectra are very close to each other. Therefore, only the IR spectrum of the second polymorph was considered in the present study. Periodic DFT computations of α 2 -polymorph led to the appearance of imaginary frequencies. This problem is usually solved by reducing the space symmetry of a crystal [77,78]. Reducing the space symmetry group to P1 allowed us to get rid of imaginary frequencies of α 2 -polymorph. An accurate interpretation of the experimental spectrum assumes knowledge of the relative stability of polymorphs α 2 and β. This requires the calculation of the sublimation enthalpy, which is very cumbersome in the case of conformationally mobile molecules [79]. In addition, the error in the calculated values can reach 15 kJ/mol [80]. Such an accuracy of calculations is hardly suitable for describing "very close points in the crystal structure landscape" of crystals with conformationally mobile molecules, e.g., see [81].

Conclusions
The paper presents the synthesis of 2-[(E)-(phenylimino)methyl]phenol (SA) and its two deutero-derivatives (SA-OD and SA-C 6 D 5 ). 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-C 6 D 5 made it possible to show the influence of deuteron replacement in the remote group (aldimine ring) on the hydrogen bridge stretching vibrations ν 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 agreement between the measured and calculated spectra, allowing for spectroscopic distinction between polymorphs α 2 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-C 6 D 5 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 α 1 and α 2 forms) were assumed to co-exist in the mentioned temperature range.