*Article* **Polymorphism and Conformational Equilibrium of Nitro-Acetophenone in Solid State and under Matrix Conditions**

**Łukasz Hetma ´nczyk 1, Przemysław Szklarz 2, Agnieszka Kwocz 2, Maria Wierzejewska 2, Magdalena Pagacz-Kostrzewa 2, Mikhail Ya. Melnikov 3, Peter M. Tolstoy 4 and Aleksander Filarowski 2,5,\***


**Abstract:** Conformational and polymorphic states in the nitro-derivative of *o*-hydroxy acetophenone have been studied by experimental and theoretical methods. The potential energy curves for the rotation of the nitro group and isomerization of the hydroxyl group have been calculated by density functional theory (DFT) to estimate the barriers of the conformational changes. Two polymorphic forms of the studied compound were obtained by the slow and fast evaporation of polar and non-polar solutions, respectively. Both of the polymorphs were investigated by Infrared-Red (IR) and Raman spectroscopy, Incoherent Inelastic Neutron Scattering (IINS), X-ray diffraction, nuclear quadrupole resonance spectroscopy (NQR), differential scanning calorimetry (DSC) and density functional theory (DFT) methods. In one of the polymorphs, the existence of a phase transition was shown. The position of the nitro group and its impact on the crystal cell of the studied compound were analyzed. The conformational equilibrium determined by the reorientation of the hydroxyl group was observed under argon matrix isolation. An analysis of vibrational spectra was achieved for the interpretation of conformational equilibrium. The infrared spectra were measured in a wide temperature range to reveal the spectral bands that were the most sensitive to the phase transition and conformational equilibrium. The results showed the interrelations between intramolecular processes and macroscopic phenomena in the studied compound.

**Keywords:** polymorphism; isomerization; phase transition; nitro group; matrix isolation; IINS; FT-IR; Raman; X-ray; NQR; DSC; DFT
