Ácido adipico em condições extremas de temperatura e pressão

Abstract

Dicarboxylic acids are organic compounds characterized by the presence of two carboxyl groups (COOH) in their molecular structure, which are responsible for their acidity. These compounds are found in more complex organic systems and play important roles in various chemical and biological processes. In addition, they attract significant interest due to their wide applicability in different industrial fields, making it essential to understand their physical and chemical properties under varying temperature and pressure conditions. In this context, this work investigates the structural behavior of adipic acid C6H10O4 under extreme temperature and pressure conditions through a combination of Raman spectroscopy, powder X-ray diffraction, and density functional theory (DFT) calculations. Initially, vibrational characterization was carried out under ambient conditions, allowing the classification and assignment of the observed modes with the support of computational calculations performed on the unit cell. At low temperatures, the sample was cooled down to 10 K, where a phase transition previously reported in the literature was observed in the range between 135 K and 130 K, accompanied by crystal fragmentation and significant changes in the Raman spectrum. In order to better understand these results, powder X-ray diffraction measurements were performed, revealing phase coexistence below the transition temperature, persisting down to approximately 10 K. Under high pressures, the sample was investigated up to values above 7 GPa in two independent Raman spectroscopy experiments. Significant changes in intermolecular vibrational modes, together with an increase in the number of modes above 1.6 GPa, indicated a phase transition associated with a reduction in the symmetry of the crystal structure. At higher pressures, variations in the intensities of internal modes suggest possible conformational changes of the molecules within the unit cell. The results obtained highlight the strong dependence of adipic acid on thermodynamic conditions, contributing to the understanding of phase transition mechanisms in this molecular system.