2.1.4. Nitro Group Mode

*Solid state*. The bands most sensitive to polymorphic states were assigned to the nitro group vibrations. The bands at 1533 cm<sup>−</sup><sup>1</sup> (νas (NO2)), 1346 cm<sup>−</sup><sup>1</sup> (νsym (NO2)), 900 cm<sup>−</sup><sup>1</sup> (δ(NO2)) in the Raman spectrum and at 899 cm<sup>−</sup><sup>1</sup> (δ(NO2) in the IR spectrum shifted noticeably upon transitioning from polymorph **I** to polymorph **II** (1540 cm<sup>−</sup>1, 1357 cm<sup>−</sup>1, 881 cm<sup>−</sup><sup>1</sup> in Raman and 894 cm<sup>−</sup><sup>1</sup> in IR spectra, Figure S7, SM). A large increase in intensity of the band at 1346 cm<sup>−</sup><sup>1</sup> was also detected in the Raman spectrum upon transition from polymorph **I** to polymorph **II**.

*Matrix condition.* Two intense bands at 1550 cm<sup>−</sup><sup>1</sup> and 1539 cm<sup>−</sup><sup>1</sup> as well as two bands at 1356 cm<sup>−</sup><sup>1</sup> and 1303 cm<sup>−</sup><sup>1</sup> observed in IR spectrum registered under the matrix condition (Figure 4) were assigned to asymmetric and symmetric vibrations of the nitro group of conformers **B** and **A**, respectively. As for the solid state, the bands assigned to conformer **B** (cf. IR spectra obtained under the matrix condition and in the solid state, Figure 4 and Figure S7, SM) were absent in IR and Raman spectra.

#### *2.2. Spectral Changes under Phase Transition in the Solid State*

Regarding phase transition, no significant band splitting during the transition from one phase to the other was observed in IR spectra measured in the middle spectral range. However, the IR spectrum in the far-infrared range demonstrated visible changes (Figure S8, SM). At higher temperatures (300–120 K) there were single bands at 454, 409, 350 and 182 cm<sup>−</sup>1, which split into doublets at the temperature below the phase transition. The IR spectra of polymorph **II** were measured to verify the results, which did not reveal the splitting of the abovementioned bands in the far-infrared range for the polymorph without the phase transition. The PED analysis showed that bands at 454, 350 and 182 cm<sup>−</sup><sup>1</sup> belonged to multicomponent modes (Table S2, SM). The intense band at 409 cm<sup>−</sup><sup>1</sup> in the IR spectrum was assigned to νσ (OHO) vibration due to a strong reduction of the intensity upon deutero replacement. The reliability of the assignment was also proved by IINS studies (Figure 5).

For a deeper insight into the vibrational spectra, isotopic effect and conformational polymorphism we studied the IINS spectra of **CNK** and **CNK-OD** within the 1200–0 cm<sup>−</sup><sup>1</sup> range in the solid state (Figure 5). These spectra showed the presence of two polymorphic forms at low temperature and verified the correctness of the assignment of the vibrations of the hydroxyl group and the hydrogen bridge (<sup>ν</sup>σ (OHO)).

Most bands of the measured IINS spectra of both isotopologues were doublets (Figure 5; in the figure, the doublets are marked by filled squares, circles and triangles). In the papers by Tomkinson [32] and Margues [33], it is stated that the split bands in the IINS spectra occurred as a result of two crystallographically and energetically non-equivalent modes. The doubling of bands in the IINS spectra was also supposed to result from the Davydov effect [34], which is the separation of energy levels ascribed to the same vibration due to the presence of several interacting molecular entities in the unit cell. Therefore, the splitting of the bands in the IINS spectra in both isotopologues was due to the presence of two polymorphic forms at 10 K. Three bands at 949, 916 and 895 cm<sup>−</sup><sup>1</sup> were observed within the range of 1000–850 cm<sup>−</sup>1, three of which were the result of th overlapping of two doublets: 949/916 cm<sup>−</sup><sup>1</sup> (circles) and 916/895 cm<sup>−</sup><sup>1</sup> (triangles). The doublet at 895/916 disappeared upon deuteration (cf. spectra **CNK** and **CNK-OD**, Figure 5) and appeared as a very small doublet at 656 and 676 cm<sup>−</sup>1. According to the calculated ISR coefficient (ISR = 1.35) and the PED analysis (Table S5, SM), these bands were assigned to the γ(OH) and γ(OD) out-of-plane bending modes. The values of the γ(OH) bands of IR and IINS spectra were compared in a qualitative way with the values obtained from the correlation R(OO) = 3.01 + 0.0044 × 10−<sup>4</sup> γ(OH) (for OHO hydrogen bridges longer than 2.4 Å) presented in reference [35]. The position of the γ(OH) band shifted towards the high frequencies alongside the strengthening of the hydrogen bond (823 cm<sup>−</sup><sup>1</sup> in matrix condition <860 cm<sup>−</sup><sup>1</sup> in the solid state). This fact supports the tendency shown in the review by Novak [36] for medium-strong intramolecular hydrogen bonding.
