**2. Results and Discussion**

At first, the studies dwelt on the DFT calculations of the potential curves to detect stable and metastable states of the molecule according to the concepts presented in the review paper by Bernstein [2]. The information about the energy difference between the global and local minima as well as the height of energy barriers made it possible to predict the presence of a particular conformer or polymorph depending on the environment. For the studied molecule it was logical to consider two possible intramolecular processes— reorientation of the hydroxyl group and rotation of the nitro group (Scheme 1). Therefore, the dependencies of the potential energy on the turning angle of the hydroxyl and acetyl groups were obtained by DFT calculations, which were performed under step-by-step

changes to the torsional angle while all other geometric parameters of the molecule were optimized. The calculation of potential energy dependence on the Θ angle of turning the nitro group is given by the equation Δ*E* = *f*(Θ), where Δ*E* = *E*min − *E*i; *E*min is the minimal energy of the system; and *E*i is the energy of the system for each fixed Θ angle). The result demonstrated that the energy barrier was rather small and equaled 0.78 kcal/mol when the Θ angle equaled 0 degrees (Figure 1).

**Figure 1.** The calculated (B3LYP/6-311++G(2d,2p)) potential energy curves for the intramolecular hydrogen bond transformation (picture in the centre) and the nitro group rotation (picture in the left related to conformer B and picture in the right related to conformer A).

Such a result may seem unconvincing because the torsional angle at 0 degrees (a flat molecule) is usually characterized by minimal potential energy because of π-electronic coupling between the nitro group and phenyl moieties. However, the studied molecule displayed a significant steric repulsion between the oxygen of the nitro group and the oxygen of the hydroxyl group, which counteracts the π-electronic coupling. Such steric repulsion results in the appearance of an energy barrier (Δ*E* = 0.78 kcal/moL) at Θ = 0◦ (Figure 1). Notably, this barrier is not large; thus, the nitro group can easily change its position with respect to the phenyl ring. The process of reorientation of the hydroxyl group leading to the transition from conformer A to conformer B is more complicated and is presented in Supplementary Materials (Supplementary Materials, Figure S1). Judging from the calculated energy barrier of 14–15 kcal/mol, one expected the existence of both conformational forms at a reasonable temperature [21]. Taking these findings into account, we assumed that the studied compound featured polymorphism brought about by the rotation of the nitro group, or by a significant conformational change introduced by the reorientation of the hydroxyl group.

We performed comprehensive studies to trace polymorphs, phase transitions and conformational change. Two polymorphs of the **CNK** compound were obtained by slow recrystallization from methanol (polymorph **I**) and fast re-crystallization from chloroform (polymorph **II**). X-ray studies of polymorphs **I** and **II** showed that they crystallized in a Pccn (T = 200 K) and P21/c (100 K) space group, respectively. The comparison of the structures of the polymorphs clearly showed the difference in the position of the nitro group (Figure 2 and SM Figures S2 and S3). The disoriented position of this group in polymorph **I** points to significant dynamics in the solid state. The crystal cell of polymorph **II** is characterized by a more defined orientation of the nitro group. The nitro groups of polymorph **I** are able to rotate, but the nitro groups of polymorph **II** are not since in the crystal cell of polymorph **II** the molecules are packed in a way that turns the nitro groups and blocks the rotation. The studies of both polymorphs were carried out by DSC to detect a phase transition. The DSC measurements of both polymorphs showed the presence of a phase transition for polymorph **I** at 109.8 K (cooling)/114.5 K (heating) (Figure S4, SM) and the absence of a phase transition for polymorph **II**. The phase transition is reversible and the transformation is enantiotropic.

**Figure 2.** Selected view of the molecular packing of polymorph **I** (**left side**) and polymorph **II** (**right side**).

The comparison of the structural data and the packing of the molecules in the crystal cell of both polymorphs made it possible to conclude that the phase transition was conditioned by the following phenomenon. A decrease in temperature tightened the packing of the molecules in the cell and, therefore, caused a stronger interaction between the nitro groups. Polymorph **II** featured "jagged" nitro groups, which evoked the stable position of molecules in the crystal cell upon temperature decrease. Polymorph **I** lacked this phenomenon, having the possibility of a looser nitro-group rotation at room temperature, which stopped when the temperature was lowered. The decreasing temperature led to stronger interactions between nitro groups and shifted the molecules towards each other, triggering the phase transition. Such a conclusion was verified by comparing the packaging of **CNK** to that of the structurally similar 5-methyl-3-nitro-2-hydroxyacetophenone (**MNK**) [22]. For **MNK,** the nitro groups were oppositely directed and not able to interact strongly, so they did not provoke tensions in the crystal cell. Therefore, the structurally close **MNK** did not exhibit phase transition and polymorphism (cf. Figure S5, SM).

The phase transition in **CNK** was detected on the basis of NQR measurements. NQR was successfully used in the research of compounds with intermolecular [23] and intramolecular hydrogen bonds [24]. NQR studies revealed that the 35 Cl signal shifted to high frequencies (from 35.85 to 36.5 MHz) when the temperature fell from 300 to 120 K (the temperature of phase transition), whereas the signal was quite stable at temperatures below the transition state (Figure 3). A similar trend was observed for 1,3-diazinium hydrogen chloranilate monohydrate [25], morpholinium hydrogen chloranilate [26] and *p*-dichlorobenzene [27].

After comparing the structural data and the crystal packing of both polymorphs we concluded that the polymorphism and phase transition were conditioned by the position of the nitro group. Based on experimental data, this conclusion was in accordance with theoretical predictions.

**Figure 3.** The 35 Cl NQR spectra (**left side**) and the temperature dependence of the 35 Cl NQR frequency (**right side**) of 5-chloro-3-nitro-2-hydroxyacetophenone.

#### *2.1. Detection of Polymorphs in the Solid State and Isomers under the Matrix Condition by Spectroscopic Methods*

To detect the spectral bands that are the most sensitive to polymorphic and conformational changes as well as to phase transition, we performed an analysis of the vibrational spectra measured in solid state and under the matrix condition. To that end, IR, Raman and IINS spectra of the studied compound and its deuterated derivative (OH → OD) were measured in the wide spectral and temperature ranges (50–4000 cm<sup>−</sup>1, 300–5 K, Figures 4 and 5 and SM Figures S6–S8). The analysis of the spectra and the assignments of the bands were based on DFT and PED calculations (Tables S2–S5, SM). Below, the description of the spectra measured in the solid state and the matrix isolation condition is presented, on the basis of which the isomeric equilibrium analysis was carried out.

**Figure 4.** Matrix isolation IR spectra (in Ar) of 5-chloro-3-nitro-2-hyroxyacetophenone (black line) and its deuterated derivative (red line).

**Figure 5.** IINS spectra of **CNK** polymorph **I** (red line) and its deuterated analogue (black line). The IINS spectra were aligned and normalized in 0–250 cm<sup>−</sup><sup>1</sup> and 250–1250 cm<sup>−</sup><sup>1</sup> regions, separately.

## 2.1.1. The ν(OH) Stretching Mode

The adequate assignments of the bands of stretching (ν(OH)), in-plane (δ(OH)) and out-of-plane (γ(OH)) bending modes of the hydroxyl group, as well as the isotope effects of these vibrations caused by the replacement of the bridged hydrogen by deuterium (OH → OD) was completed.

*Solid state.* When comparing the infrared spectra of deutero-(**CNK-OD**) and nondeutero-(**CNK**) derivatives of the studied compound, the band at 3000–2100 cm<sup>−</sup><sup>1</sup> in the IR spectra recorded in the solid state was assigned to the ν(OH) stretching mode due to its shift to 2100–2000 cm<sup>−</sup><sup>1</sup> after deuteration (Figure S6, SM). It was necessary to underline that the measured infrared spectra in the solid state did not reveal the intense band at 3100 cm<sup>−</sup><sup>1</sup> that was assigned to the stretching vibration of the hydroxyl group, which is hydrogen bonded with the nitro group (cf. spectra of **CNK** and *o*-nitro-phenol [28]). This fact confirmed the absence of conformational form **A** of the studied compound in the solid state. The result agreed with the presented X-ray study.

*Matrix condition.* The analysis of the ν(OH) band in the infrared spectrum measured under the matrix conditions does not provide a clear proof for the presence of two conformers. The reason is the overlapping of ν(OH) bands of both conformers and ν(CH) bands. However, two bands at 2185 cm<sup>−</sup><sup>1</sup> and 2350 cm<sup>−</sup><sup>1</sup> appear in the spectrum of **CNK-OD**; they are assigned to the ν(OD) vibration of conformers **A** and **B** (Figure 4), respectively. This result manifests the presence two conformational forms under the matrix condition.

#### 2.1.2. The δ(OH) and γ(OH) Bending Modes

*Solid state.* The in-plane bending mode (δ(OH)) was hard to analyze because of an uncharacteristic vibration in the solid state. According to PED analysis of the spectra calculated by the DFT method, this vibration was strongly coupled to the stretching vibrations of the phenyl ring (1563 and 1433 cm<sup>−</sup>1, Table S2, SM).

A broad band at 860 cm<sup>−</sup><sup>1</sup> for polymorph **I** and the band at 838 cm<sup>−</sup><sup>1</sup> for polymorph **II** (Table S2 and Figure S7, SM) were assigned to the out-of-plane mode of the hydroxyl group (γ(OH)) in IR spectra measured in the solid state. The bands at 860 cm<sup>−</sup><sup>1</sup> and 838 cm<sup>−</sup><sup>1</sup> narrowed down drastically under the deutero replacement, and new bands arose at 628 cm<sup>−</sup><sup>1</sup> and 624 cm<sup>−</sup><sup>1</sup> in the **CNK-OD** spectra (Figure S6, ESI). These bands were assigned to the γ(OD) mode according to ISR = 1.35 (ISR—isotopic spectroscopic ratio).

*Matrix condition.* As for in-plane and out-of-plane bending modes, a more distinct picture (due to the absence of the overlapping bands) was observed in IR spectra measured under the matrix condition. In these spectra, obvious changes to two bands at 1269 cm<sup>−</sup><sup>1</sup> and 1166 cm<sup>−</sup><sup>1</sup> appeared after deuteration. They completely disappeared after deuteration and new bands appeared at 959 cm<sup>−</sup><sup>1</sup> and 895 cm<sup>−</sup><sup>1</sup> in the **CNK-OD** spectrum (Figure 4). According to the observed changes and the calculated isotopic ratio (ISR = 1.32), these two bands were assigned to the δ(OH) and δ(OD) modes of conformer **B** and conformer **A**, respectively (Table S3, SM). Rather broad and weak bands at 823 cm<sup>−</sup><sup>1</sup> and 719 cm<sup>−</sup><sup>1</sup> completely vanished and turn up at 619 cm<sup>−</sup><sup>1</sup> and 534 cm<sup>−</sup><sup>1</sup> after deuteration (ISR = 1.33), the bands having been assigned to the γ(OH) and γ(OD) modes of conformer **B** and **A**, respectively. The results presented above are in agreemen<sup>t</sup> with the data based on PED analysis (Table S3, SM) as well as the data obtained earlier for 5-methyl-3-nitro-2- hydroxyacetophenone [29].

#### 2.1.3. The ν(C = O) Stretching Mode

It is noteworthy that the stretching vibration of the carbonyl group (ν (C = O)) was the most sensitive to the conformational equilibrium [30,31]. According to IR and Raman spectra measured in the solid state (Figure S7, SM), only one band was observed in the 1800–1600 cm<sup>−</sup><sup>1</sup> range at 1650 cm<sup>−</sup><sup>1</sup> and was assigned to ν (C = O) mode (PED analysis, Table S2, SM). Interestingly, the ν (C = O) bands of both polymorphs were nearly the same, and therefore, demonstrated very little sensitivity to polymorphic changes. However, IR spectra under the matrix condition were characterized by two bands at 1700 cm<sup>−</sup><sup>1</sup> and 1667 cm<sup>−</sup>1, which were assigned to the ν (C = O) vibrations of conformers **A** and **B** (Figure 4), correspondingly. This statement is in accordance with our previous studies of the methyl derivative of *o*-hydroxy acetophenone [29].
