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

Area of Science:

• Solid-state chemistry
• Molecular spectroscopy
• Computational chemistry

Background:

• Understanding molecular behavior in different solid forms is crucial for material science.
• Polymorphism and conformational changes significantly influence a compound's physical and chemical properties.
• Nitro-derivatives of acetophenone are important in various chemical applications.

Purpose of the Study:

• To investigate the conformational and polymorphic states of a nitro-derivative of o-hydroxy acetophenone.
• To elucidate the relationship between intramolecular processes and macroscopic phenomena.
• To analyze the impact of the nitro group's position on the crystal structure.

Main Methods:

• Density functional theory (DFT) calculations for potential energy curves and conformational barriers.
• Experimental techniques including X-ray diffraction, IR and Raman spectroscopy, IINS, NQR, and DSC.
• Argon matrix isolation for observing conformational equilibrium.
• Variable-temperature infrared spectroscopy to identify phase transition-sensitive bands.

Main Results:

• Two distinct polymorphic forms were obtained via controlled evaporation methods.
• A phase transition was identified in one of the polymorphs.
• Conformational equilibrium due to hydroxyl group reorientation was observed and analyzed using vibrational spectroscopy.
• DFT calculations provided insights into conformational change barriers and nitro group influence on the crystal cell.

Conclusions:

• The study establishes a link between intramolecular dynamics (conformational changes) and macroscopic properties (polymorphism, phase transitions).
• The nitro group's position critically affects the crystal structure and overall behavior.
• Vibrational spectroscopy is a powerful tool for interpreting conformational equilibrium and phase transitions in such compounds.