**1. Introduction**

The aim of this paper is to investigate polymorphic and conformational states of the nitro-derivative of *o*-hydroxy acetophenone. The study explains the influence of the intramolecular hydrogen bond on the phase transition and conformational equilibrium. The expected intramolecular dynamic processes in the *o*-hydroxy acetophenone molecule are the rotation of the nitro group and isomerization of the hydroxyl group (Scheme 1). These processes define such macroscopic phenomena as polymorphism, phase transition and the existence of stable isomers under different conditions. The study of conformational isomerism is a very important issue in modern chemistry for determining and modelling the physical–chemical properties of new materials [1–4] and pharmaceutical compounds [5–8].

**Citation:** Hetma ´nczyk, Ł.; Szklarz, P.; Kwocz, A.; Wierzejewska, M.; Pagacz-Kostrzewa, M.; Melnikov, M.Y..; Tolstoy, P.M.; Filarowski, A. Polymorphism and Conformational Equilibrium of Nitro-Acetophenone in Solid State and under Matrix Conditions. *Molecules* **2021**, *26*, 3109. https://doi.org/10.3390/molecules 26113109

Academic Editor: Mirosław Jabło ´nski

Received: 11 May 2021 Accepted: 21 May 2021 Published: 22 May 2021

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It must be stressed that an intramolecular hydrogen bond strongly influences the isomerization equilibrium [9,10]. For deeper insight into the hydrogen-bonding effect on the polymorphic states and isomerization we applied a wide variety of research methods (DFT, X-ray, DSC, NQR, IINS, IR and Raman) in different environments and over a wide temperature range. It is worth noting that the method of matrix isolation was unique in its ability to trace metastable states and help interpret complex reactions [11–14]. Moreover, investigations of objects with hydrogen bonding by IINS [15–18] and NQR [19,20] techniques are useful.

**Scheme 1.** Schemes of intramolecular hydrogen bonding isomerization (**A**,**B**) and nitro group rotation (**C**) for 5-chloro-3-nitro-2-hydroxyacetophenone (**CNK**).

In this paper the research methodology followed the following sequence: First, DFT calculations were performed to detect stable and metastable states of the studied molecule, 5-chloro-3-nitro-2-hydroxyacetophenone (**CNK**). The first stage of the studies predicted that **CNK** would crystallize into two polymorphic forms; therefore, its structural properties were analyzed by X-ray diffraction at different temperatures, and both polymorphs were studied by the DSC method to detect phase transition, which was found in one of the polymorphs and verified by 35 Cl NQR measurement. Spectral properties of both polymorphs were investigated by IINS, IR and Raman, and computational (DFT) methods in different states to obtain exhaustive information about which vibrational bands (as well as their assignments) were the most sensitive to the phase transition. The assignments of the spectral bands were accomplished on the basis of the H/D substitution of bridge hydrogen (OH → OD) and Potential Energy Distribution (PED) analysis. Spectral infrared studies of the two isotopologues (OH and OD) showed the presence of two conformers (A and B, Scheme 1) under the argon matrix condition.
